You'll slow you car down!. Mines much quicker off the lights with the 16's on it (feel properly quick!).
On 18's though the handling is better through bends but comfort is thrown out of the window!. Short stiffer sidewalls stop tyre squirm and the wider tyres give a bigger foorprint so grip in theory should be better.
However buy ***** tyres and your car will handle badly so quality boots is a must.
I got 17's but I bout tyres with a re-enforced side wall. Not driven on 18's but I figured that if the side wall flexing is the problem with having a not so low profile tyre, then getting a re-enforced one may help. Tyres are Eagle F1 Asymmetric - 225/45 17 XL [94 Y iirc] the XL is the re-enforced code, and the load index does go up by a couple of points.
Well I cant say for sure it is the same, theoretically speaking there are loads of forces to take in to consideration, I just did not want to drop a wad of cash on new wheels, and needed new tyres anyway. I don't exactly push the limits, but the handling is good, although I am on std. suspension, so not sure if it would be the same if I upgraded teh suspension?