If you reduce sidewall, you reduce arch gap We are talking fractions here: I'm not convinced that a 285/40 would look noticeably different from a 275/35 if I'm honest, not from the side. 10/12/15mm is nothing, bearing in mind that's over the diameter so just 5/6/7.5mm top or bottom. On a car that's 4m long and 1.3m high it's going to be almost impossible to tell. I'd be more concerned about buying decent tyres than worrying about the width of a pen in arch gap.
The guys that run 19s have got it right in terms of looks, but I prefer to stick with 18s as well because I don't want the extra weight.