Geordie - perhaps itd be an idea to drive someone elses motor on 17s/18s to see how it drives?
From what Ive heard, a lot of the balance of the 172s comes from having small-ish wheels with high-ish profile tyres to make break-away more gradual at the limits of traction. Bigger wheels would increase grip but make it more snappy on the limit. I dont know how much by (hence the need for a test drive or to borrow someones rims for a day perhaps?), but only you can decide if you can live with the difference and adjust your driving to suit.
Part of the reason that I wouldnt change from 15s on the old valvers/williams is that theyre so nice and progress. I know 17s look cool and grip better, but part of my enjoyment comes from nearing the limits and playing around with them, rather than having such massive grip that you never find the limit until you dont expect it... Thats why I prefer the exs valver to drive round a twisty road rather than my stupidly low, stupidly hard-sprung RST! I just have no idea when mine will let go and throw me into a ditch!
