Luis,
I think we spoke on Sunday briefly. I was in a non S yellow boxster that was well, quite quick
1) 17" wheels, and their tires, will be cheaper than 18" wheels and tires. Could be as much as $50-70 per tire cost difference. Plus, you can pick up 17" rims for $600-1000 vs twice that or more for 18"s.
2) What size tire do you want to run. What brand tire do you want to run.
3) While the side profile may be taller on a, for example 255/40-17 vs a 265/35-18, you will find the overall circumference is very close. Then again, I can't say I call a 40 series tire a balloon
You will find that, say a Khumo XXX comes in 225 and 255 in 17", but a Yokohama or Toyo might make a 235/255 combo. Small differences but something to consider.
Looking back, I wish I went 245's all around because I would prefer to get the rear of the car to rotate more. However, some shops don't want to squeeze a 245 on a 7.5" wide rim. So, in my case I am running 225/275-18s (stock is 225/265) but notice a touch more understeer than I desire.
Its all good fun but remember the best improvement to make is SEAT TIME. Once you learn to drive well, then the other enhancements can make you even quicker.