Seen a set of GXP wheels and tires on Ebay. Would it be a worthwhile upgrade from 17'* to 18.* ?

I'm going to need to be buying tires soon either way

i would for sure, those wheels are smooth

If the profile changes too much you will need either a custom pcm (intense) or a set of transmission reluctor gears to re-calibrate the speedo.
The bigger wheels are a little harder to move so your 0-60 times will get a bit longer and some say the ride is a bit rougher too.

As long as the overall diameter of the tire is the same you should have no problem.The ride is better on my car than it was stock

If you do upgrade, I might be interested in your old ones if they are the chrome ones.

Definately have a more agressive feel to your ride. Little rougher also, just on big bumps. Overall ride is pretty smooth like stock.

I believe they actually use my tire.. it'* a 235-50zr18 .. and my ride feels smooth as ever..

Go for it expect a slightly rougher ride with the shorter sidewall but with a more aggressive turn in....if you have the chrome rims I would bet that weight wise they would be pretty similar. Youre factory tire with 235/55/17 is 27.18" OD and the 235/50/18 is 27.25" OD so dont worry about the speedometer error it wont be big enough to even notice, most manufacturers allow for up to a 5% difference in tire OD just because of the factory tire size differences. .07" of differences is WELL within 5% OD difference so you would be fine...plus those GXP rims are friggin tite dawg!