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19 inch is better looking but heavier thus hurting performance with unsprung weight?
yes
19's ride harder but offer a flatter surface for grip?
some say. disagree on the grip. most race cars do not ever run a 30 sidewall, so I call BS
Will 18's accommodate big brake upgrade kits?
of course, but it depends on the wheel
Having said this I think as long as the 18's will fit brake upgrades it's the better rim for the Z4M?
Your opening a can of worms with this question. For performance, yes.
Is the 18'x9.5 rim the best choice? I'm think 255's upfront and 285's rear.
for those tire sizes, I think 18x9 and 18x10 wheels. That is an aggressive rear, so you will need to find out the exact tire, as well as rear offset (et22) that will work. You also, may need to increase rear camber and roll fenders if you want to go that big in the rear. Not a big deal, but depending on your needs, it will be easier to put 275's and call it a day.
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