What I said in another thread earlier.
I had 17's on, apart from the scraping a bit on full lock, I never noticed much diffference. I think it comes down to personal choice and more importantly how you drive. For normal day to day driving I never noticed a thing, as for losing speed etc etc, I had nothing to compare it against as it came with 17's on. I put 16's on and all I really noticed was no scraping on full lock and none going over speed bumps (my car is 60mm lower). That said you get a lot of responses, saying it messes up your handling etc, only suggestion is to try some 17's if possible and base it on your own opinion. I only took mine off because they needed refurbing, I bought the 16's as a temporary measure. I just never found the time to refurb them, then someone offered to buy them and seen as I am now selling my car in the New Year I sold them.