[PrematureApex]

255/40 is a stock size (225/45F, 255/40R), and basically the same diameter as a 225/45. Currently, I have 255/40s in the rear with a 15mm spacer on 9" 40ET wheels from my E46 M3. So that puts it around a 25mm offset. I don't think you'd want to go much less in that department, as you'll start to stick a bit too far outside the fenders.

A 35 series would be fine as well, and as mentioned, would help gearing... but I wouldn't for a couple of reasons:

1. The ride comfort/wheel protection will be that much worse, and
2. The fenders of this car are huge. A 255/35 is going to fill in the massive spaces even less.

I'd buy 35ET wheels, and use spacers to fine tune the fit to your liking.

245/40 or 245/45 wouldn't be an awful way to go either.