Rob,
You're right, the 15" tires won't have the same performance. But if you want a more original look, that's what you should go with. If you are only going to street cruise the car, it really won't be much of a problem. If you really want the performance, 17" is the only real choice. 16" tires are hard to find.
Did you really think the 17" on my car didn't look right? Honestly it is one of the things I get the MOST compliments on about the car. So many people love my rims. I think 17" wheels look great. They don't overwhelm the car, and the handling is awesome. The slight trade off of harder ride and slight wandering is well worth it. I have Firestone Firehawk SZ50 EP on mine. 245/45/17 up front, 275/40/17 in the rear.
A lot of it depends on how "real" you want the car to look. To me, the entire car isn't an original '60's Cobra, so I don't care if it doesn't look exactly the same.
Steve
Cobra Revolution June 7th, Valley Forge Park
www.legendarycobra.com