Right, the endplates wouldn't make a huge difference and increasing span would be more beneficial. However, I don't think endplates would have been out of the question just because the wing is integrated. On the performance side of things it's a case of diminishing returns, but this is a $400,000 or so car.
I wouldn't think the car needs to raise the wing for stability in the first place. 200+ mph stability can be achieved without a wing. If the raised wing truly is necessarily it probably means there is a decent amount of front downforce that needs to be balanced out. If you're really being practical, you could just use a fixed wing at that point. If top speed was a goal you could perhaps get away with active angle of attack adjustment. A wider fixed wing with endplates would be less draggy for the amount of downforce produced anyway, and if you kept the same area by shrinking the chord you could increase your pitch moment by moving the wing back and achieve desired balance with even less drag.
