The optional 17's come with Goodyear Eagle F1 tyres, which are supposed to be one of the better tyres. Being directional though, you notice the increase in road noise from the standard wheels. The increase in grip is incredible (from both the wider rubber and better quality), but so is the increase in wheel tracking on bad surfaces and heavier steering.
All things considered, including looks, getting the optional 17's was the best addition to the car so far, in my opinion. I do have to agree with Zetec though, in that I do like the looks of the ST170 17 inch wheels alot, and would have probably taken a set over the 7 spokers.
Cheers..... Tim