The car gets slower when you change the width of the alloy and of the tyre. The whole circle, if it is the same dimension as before, you have no problem. Of course there is also the weight. But think of the magnesium alloys, which are almost the half weight of the stock, even if you put more width, you will have no prob from the loss of weight.
I had stock, 195/60/15 with 6,5x15. I have put, 205/45/17 with 7x17(ST rims). I have lost 4 horses on the wheel. Bad for me, but I'll put many horses in the future...
-The most important, those rims on the RS are very good!! Just give the stock rims, ti get your money back.[thumb]