I currently have 18 x 8 wheels (3 piece forged) with 225/40/18 tires all around on my 328i. I'm looking to purchase an M3 within the next few months, and although I like how my 18s look I don't want to put them on if I will be decreasing the performance. The tires are only 225s, which is narrower than the stock 245s in the rear.

So out of these options, which one is the best?

-Run 18x8 with 225s all around
-Run 18x8 with 225s in the front and 245s or 255s in the rear (staggered)
-Keep the stock M3 staggered setup (225s..245s)
-Run 17x8.5 or 9 with 255s all around
-??? different route ???

If you have any other ideas please let me know. I prefer to keep my current rims because they are rare and 3 piece.

if you really like you forged wheel... you can keep them w/ the same size tires... but if say your the offset on your wheels are between 41-46 you can probably run 255/35/18 for a staggered setup....

Is running 225s all around really that bad on an M3?

I would say the handling would be better with the 225/40R18 on all four corners than it would be with the staggered tires. The wider rear tires will make the car understeer more. If you have forged wheels they are probably lighter weight as well and that would improve performance as well.