As per this thread, I fitted the new wheels yesterday........
I also changed the grooved discs that came with the 314mm brake upgrade kit I bought from Matt back to normal discs (and set of new OEM pads as well).
Visually there's not a huge difference in the size of discs compared to the 302mm I had as standard, but the calipers are somewhat larger!! The extra 12mm in diameter also helps to fill the 18s a bit more as well.
Even though there may only be 12mm difference in disc size, that equates to 8% more disc area. Along with the different calipers there is a noticeable difference in braking power compared to the 302mm setup - even though I'm still only just bedding them in.
Edit: Forgot to say. I'm very, very happy with the wheels - with how they look (always liked this wheel) but also with the ride and how they handle. I'd always been unsure about getting larger wheels as I didn't want too much of a harsher ride.
The ride with my 17s had felt a bit harsh for some time and I would get a kinda thumping noise on rough roads etc. which was driving me nuts. It'd been in to the dealers and they replaced a couple of suspension parts thinking it was them, but no difference. Went back in and they could still hear the noise but thoroughly checked everything over and couldn't fault any part of the suspension - so I thought I'd just have to live with it.
I was a bit concerned as to what the ride would be like with the 18s but its superb! The noises have gone and on rough roads it's soooo much smoother and doesn't shake you about. I can't explain it, all I can think is maybe it was the tyres! Whilst the Michelin Pilot Sport 2s are great tyres, maybe there is something about them that, on this car anyway, was causing it. I don't know - but I'm not complaining!