Rob,
Tony is right. I'd strongly recommend against putting 245s or 255s on the back of that car with the motor you are planning to put in. It's a recipe for instant disaster. That car will need 275 or 295s on the back. I think for street driving, the 15" tires will do fine if that is the look you want. The performance will be adequate for those twisty backroads. No, they won't pull over 1g on a skidpad, and yes, they'll slide around if pushed hard, but you probably won't push THAT hard on the street where it will be a continous problem. If you really want ultra high performance, you really have to go 17". So, it's really a question of do you want the "look" or the ultimate in performance? It may be hard to have your cake and eat it too here.
Steve