
Not sure I'd vary the voltage with speed though. Instead I'd add another controller very much like the FW one but with a higher target margin, say 4000, and limits of say 350V (ideally a little below the actual battery voltage) and 550V. The output from this would then be the target voltage for the boost converter controller. The idea would be that normally no boosting occurs, so minimal boost losses, but if the voltage margin drops below 4000 then the boost comes in and restores the margin. Once the boost converter runs out of volts the margin would continue to drop and once it hits 2000 the FW controller would come in as usual.
I think this may actually be more efficient overall as it would be operating in pure MTPA, rather than FW, for more of the time.