Second, less important question; if I add kWh's to my battery system by increasing the Voltage as opposed to increasing Ah's will this actually give me more range or just higher performance?
There is no free lunch.
The way that you shuffle the battery does not add or remove the amount of battery.
You could connect all 3 in series to get higher voltage. You could connect all 3 in parallel to get more amp-hours. Neither changes the amount of battery, so neither changes the amount of kWh. (kWh = Voltage x Amphours).
Energy is an amount, like how much water is in a bucket. Power is the rate at which you consume the energy you have available. It takes power (wattage) to keep a vehicle moving a certain speed, overcoming rolling and air resistance (plus any extra for acceleration or hill climbing). The power it takes to move a vehicle a certain speed is blind to where that power is coming from. It could be you pushing, it could be a gas engine, it could be an electric motor. A typical vehicle takes about 15,000 watts to maintain highway speed.
Your range (at a given speed, generally higher speeds are worse than linearly wasteful), is limited by the amount of energy you have in your battery. For how long can you sustain the flow of electricity out of your battery as needed to travel the speed you want to go. If it takes 15,000 watts to travel highway speed, and you have a 15,000 watt-hour battery, your battery will be empty in 1 hour. If you were travelling 60 miles an hour, your range will be 60 miles (how far you got in 1 hour before the battery bucket was empty).
How you shuffle the batteries does not change how much battery there is.
There are usually marginal improvements (<5%) for having higher voltage because of losses in the cabling. And, the higher the voltage the thinner your cables can be (linear, by cross section). But otherwise, what's important is giving your controller and your motor the correct voltage it was designed for, which, in this case, is a huge range, so it doesn't really matter.
Electric motors will have their speed limited by their voltage. As a motor picks up speed, it generates a voltage that cancels out the extra voltage you're giving it. The faster it spins, the more voltage. Eventually the motor makes an equal amount of voltage as the controller is giving it, and the motor stops accelerating. This doesn't depend on the load, only the RPM. The amount of voltage the motor generates as it spins depends on the way the motor is built, but they'll all be similar. Voltage is how fast you can ask a motor to spin, and the difference between that and how fast it's currently spinning is how aggressively you're asking.
Another limitation is current. As the motor is trying to spin, an amount of amps are flowing through the wires to make it spin. This depends on how hard it is to rotate the motor. If the tires are lifted, it will take very few amps. If you're towing a motorhome and climbing a hill, it will take a massive amount of amps compared to unloaded at the same speed. Amps are the answer from volts. Amps are caused by volts, so even if your battery could supply more amps, if you don't have the voltage necessary to even ask for more, you won't use any more.
At extremes, it wouldn't make sense to build a massive 1-cell battery to try to drive an EV at 4.2v. Even if there's enough energy stored in this massive battery, the motor would quickly speed up to where its voltage matches the 4.2v you're giving it, and it stops accelerating. You have extra current available (all these cells in parallel), but no way to use it because your voltage is too low.
It also wouldn't make sense to build a skinny 3000 cell battery, because each cell would choke trying to provide enough current to get the motor spinning. You have all this extra voltage (all these cells in series, a 12,000v battery), but no way to use it to spin the motor because the cells are already maxed out for current.
...
In context, we're not dealing with those extremes. We're looking at, at most, a handful of batteries and you're trying to decide which way to arrange them.
Since you know that 1 battery is sufficient in voltage to move the car to the top speed you want, there's no advantage (one caveat, later) in connecting them in series to get a higher voltage.
Since you know that 1 battery is sufficient in current to move the car to the speed (or load, mostly acceleration, towing, or top speed) you want, there's no advantage in connecting them in parallel to get more amps.
So, since there's no advantage either way, it doesn't matter, it just matters that you're adding more battery, so that you can last longer at the power rate you're consuming the energy.
If you desired better performance, you'd have to discover (maybe through experimentation) which limit you're hitting first. Either top speed (voltage), or max current draw (current), and then juggle your packs accordingly to add the thing that's holding you back.
The caveat is that your voltage might not be high enough to be asking aggressively enough for more speed (to make it chase that new top speed more aggressively). At lower speeds you might not want or need any higher acceleration (lots of low end torque), but this drops off at higher speeds. If you want faster acceleration still at higher speeds, you might want a higher voltage pack.
...
Your battery isn't directly powering your motor though. The controller is in between.
Your controller can fake lower voltage, but it (well, most) cannot fake higher voltage.
Your controller fakes a lower voltage by turning on and off rapidly. For example, if a controller is given 500v from the battery, but only wants it to appear like 250v to the motor, it just flicks the switch so that it's on 50% of the time, off for 50% of the time (or, a more complicated AC equivalent waveform based on frequency). That's how you can temper your acceleration so every speed change doesn't feel like drag racing. But, it can't be any more "on" than to be on 100% of the time. So if you feed it 250v, you can't magically ever get 500v out of it by leaving it 200% on, there is no 200% on.
What this also means is that the controller can always be given a high enough voltage to always demand a high enough speed and a high enough current from the battery.
So... generally...
the long-winded answer to your question is... if your controller can handle the higher voltage, you always give it the higher voltage, because it can fake a lower voltage (and more amps) if it needs to, but it can't fake a higher voltage than it's supplied by the battery.
...
An exception to this is the 2, 3, and 4th Gen Priuses (maybe other Toyotas?), which have a voltage booster in them, so that they can have a lower voltage battery pack but still a higher voltage given to the motor. But even those boost the voltage before they give it to the controller, so, there's no point in using them when you could just give the controller a higher voltage directly from the battery pack. Toyota made this choice (I think?) because their hybrids have such small batteries and minimum sizes on their cells that they couldn't chain any more in series to get the higher voltage any other way.
...
Also, I'm probably wrong on some of this, so, don't treat it as gospel.
you can use Field weakening to make the stator look like it generates lower return voltage - BackEMF. This is dangerous and not all inverters can do it. But delaying the magnet force you can get additional voltage credit and motor can spin faster with lesser torque. Whilst that spins motor faster inverter also consumes more current since it has to pump more amps into stator to weaken field of the rotor. This heats up stator and creates additional inefficiencies in the motor.