Did you check Kumho's site to see there wheel width range for those tires in that size?
My opinion is that the stock WRX gearing is too low... so I'd go with the 235/40-17 with is 1.9% lower than stock... the 255/40-17's would be taller than stock by about .6% so that would effectively make your gearing taller.
May I also ask way you need such wide wheels & can't be happy with the same widths front & rear?
I myself will have to settle with some Kumho 711's this year on my summer wheels. I'm saving to purchase light 16" wheels & some tires that will lower my gearing 7.2%
Thanks - Jon