Well that's the thing, IMHO going for performance and looks should be together, not trade-off each other. Form has to follow function first. Looks are also so subjective and superficial. IMHO cars that uses too big rims are actually damn ugly anyway cause they look out of proportion with the body size already! Not to mention the puny size brakes which become more obvious and gaping when using such big rims. IMHO a Myvi with 15' looks way2 much better than those with 17', and cannot imagine how ugly & stupid an 18' will look on it!
As for comfort, well people have different level of comfort/tolerance. Some can tolerate more, some not so. I personally would not go below 10cm sidewall height cause I had used before a 195/50 (9.75cm sidewall) for several years and just had enough of it and will never go to that level again. Initially might seems not so bad since the tires were still in new condition and hence softer, but starting on the 2-3rd yrs they became more uncomfortable. So I think 10cm sidewall is about the borderline, unless tire technology can improve more and more and defy the laws of physics to make thin tires but still comfortable.