I think he means arch gap as in top of tyre to to arch, which offset or 18, 19 will not help with, only lowering.
Jonty, rolling diameter stays the same with 18's, 19's or 20's, or should do, to be in line with your current rolling diameter (overall size inc tyre). Basically your tyre profile gets smaller to allow for the extra wheel diameter, so whatever your gain in wheel size, you loose in tyre size.
Use this to help you keep near the same: http://www.tyresave.co.uk/tyresize.html
This has just made my decision on going for 19s. Maybe stupid, but I never really thought about the 18s and 19s overall rolling diameter being of the same size due to variations in tyre wall depth. Thanks lol! I can live with discomfort, I want the 19s lol