Ive done a lot of trackdays on 205/45/16's with a 7.5" rim, and they grip very well and dont cause too much torque steer...

I have never had my car on 17's though so i cant compare them.

I did run back on the standard 15's with 195/50/15 for a while last year and lost all confidence in the handling of the car! It seemed to have no grip, spun the wheels too easily and i couldnt brake as hard... Couldnt wait to get back on the 16's. (Although 15's fitted with good trackday tyres would be different)

I would fit 205/45/16, and make sure you get wheels with ET35 offset.
There is much more choice of decent track day rubber in 195/50/15 though!