Fair enough mate. Well the WRX rim is 17x7 and the STi rim is 17x7.5, so I don't reckon you'll have to many dramas with the tyres looking thin on the STi's.

Essentially the width of the tyres should look exactly the same from rim to rim. Just make sure the tyres are wide enough no to be stretched over the STi rims. But with 0.5" difference I doubt it.