I went from 16 to 18 on a 1998 986. Main reason was the over read by nearly 10% on the speedo.
Went up to the next size tire and now only 2-3 % out. Any bigger and it scraped the inside of the wheel arch. Front and rear same size. For a 200hp car, I don't think it makes too much difference.
Got the rims for $160 (original Porsche) and I put on Chinese tires (after much reading of reviews) for around $700. They were more expensive than 17" but it will probably be 3-4 years before I have to change them again.
I had a track day soon after it was all fitted and the tires felt fine. May be different in another country's but the speed limit here of 100KPH, recycled plastic bags would probably do just as well for 99% of the driving.
Before the wash !!!