Did you notice much difference in ride quality between your old 16" wheels and the new 18" 108's ?
I am running on 16" 102's at the moment, and I am looking to change them to 18" wheels (again purely for looks), but I don't want to end up with a car which is overly harsh as a result.
Mine is a 2.2 on SE suspension and runflats.
Are your 108's fitted with runflats or "normal" tyres ?