My mate has 17s for his mk1 172 and when we do trackdays he swaps them about, so Ive driven the car on both 15s & 17s on the track and there IS a difference.
But, its not as bad as some people may think and the car does still handle well even with the bigger wheels/tyres on there, unlike on some other small cars Ive driven with large wheels fitted.
They dont quite give you the same sharpness and feel of the 15s and it aint quite as nimble, but I was still getting some very fast laps in with the 17s once I got used to the difference.
On the road youd never notice...