Currently have 17" on my R55, in addition to sway bar, JCW strut brace, and GP2 coil overs. I know moving up to 18" will affect handling a bit, but with the other parts I've added, is it going to affect it THAT much?

Many more wheel choices in 17", but I very much like the look of an 18" wheel...

For reference, our local clubs love to do canyon carving on the weekends. Handling is important for me, but I am willing to sacrifice a bit, so long as it is not a huge change.

 

Be aware especially from my experience. Moving up in tire will probably increase the total weight of the rim and tire. This will have a negative effect on performance and all you are doing is changing the contact pattern of the tire on the ground. It will only alter the pattern of the footprint/contact point and will not increase the contact size. You can look at my prior post on tire size. I went from 15 to 17 then finally settle on 16s. Good luck!

 

I would stay with the 17s. Food for thought: the GP2 was supplied with 17s...

 

I'd stick with the 17s more options and usually cheaper

 

Aside from coilovers I have nearly the exact same set up on my Clubman. Going with 18"s doesn't necessarily mean more weight if you choose a light weight forged rim. Pending the offset you can increase lateral handling too. Tires can also play a huge role in handling with the proper aspect ratio.

 

I would stick with the 17" wheels. I had 18's on my 2008 clubman s, it handled incredible, but knocked your filings out over a pothole. For me it's always function over fashion. Save your $$.