Well undoubtly its slower with the bigger rims...
kinda the price ya pay for the look. When someone says 'luxury' rim its a euphamism for 'heavy ass' rim.
I put my exhaust, springs and wheels on all at the same time. The power gains with an exhaust were minimal to begin with but the heavier wheels comepletely squashed any semblance of feeling faster, other than the beefier engine note.
As somewhat of an unspoken rule, Cars with short overhangs (IS, New Celica, Integra) follow a pattern. The bigger the rim, the lower you've gotta drop it. This isn't just some ghetto ricer thing either. Honestly an Integra with 17"s will look nasty if there's any wheel gap. In fact, 17"s are plenty big for that car, if the rear isn't tucked at least a little (like the E36 M3) Its not gonna look as good as it could.
The IS has a modest wheel gap to begin with, and looks okay with stock wheels... there's a bit more rubber sidewall there. With 18"s and a 40% sidewall tire, you're beggin for a drop. Don't get me wrong, I love the way your car looks, but make performance springs a priority (if not adjustable coilovers). I think your car would look great with about .5 to 1.0 finger's gap between the tire and wheel well.
All of this of course is just my humble opinion.