+1 makes sense, maybe +2, never 20's on a 328!
I disagree that 17's are not a good option. You get less sidewall flex and the car is more responsive. If you've got the sport package and you drive in a spirited manner (I think that's the correct BMW corporate way to put it.) the +1 size is good. Yes ride suffers a little, but that's a choice for each driver.
22" rims on a Hummer? Stupid as can be. You couldn't feel sidewall flex on the thing if it had soft tires on 15" rims. I roll my eyes at Honda Civics with wings stolen from 747's too. They'd actually improve their performance spending half as much money on a K&N cold air intake.
I went from 19" to 18" on my e46 M3 and I thought the tires I bought were horrid. Lord it felt squishy in comparison. I quickly got used to them, and the handling was still great, just a tiny bit of flex as you set it in the corner if you were really moving. Prior Cars: 05 M3 SMG, 04 Audi S4, 97 740il, 95 M3 Coupe.
2007 328xi coupe Steptronic, Premium, Sport CWP.