i have been doing some research on this topic as i will no doubt look to purchase an m coupe in the future.
one of the things i have noticed is the decidedly lack of aftermarket products available for the z4 m coupe.
i will try and break them down, feel free to add anything that u know of. i will concentrate mainly on performance mods, as im not really interested in bodykits (u can go to most of the main tuners for this such as ac, hartge, hamann)
ecu- revozport have an e46 m3 software upgrade which claims to give 15hp, but costs about $1200. im not sure if its compatible tho. does anyone know if the m3 software upgrades can be carried over to the m coupe, i think i read somewhere they are not the same.
dinan also offers m3 software as well, but it seems that they only have minimal gains.
evotech offers a reflash
http://www.dynocomp.com/bmw.php?view=bmw
intake- gruppe m have a nice cf unit, but costs a nice chunk of change, about $1200. my brother has a gruppe m on his evo9, looks the biz, and he gets a nice induction roar as well as excellent power gains from it.
k&n have a drop in filter unit, i doubt any power gains, but its reusable.
afe make an intake, but im not sure if it fits the m coupe
exhaust- supersprint and remus are the only major exhaust guys that i have come across that have developed a system for the m coupe. it seems that ac and hamann just rebadge the above mentioned brands and add fancy tips on them and call them their own.
milltek sport, who i regard as one of the better exhaust manufacturers, have emailed me and said that atm, they have no plans to develop a unit as demand is low
also, with ss and remus, are their units only cat back? do they offer performance headers, and what about 100 or 200 cell high flow cats?
clutch- unlock valve from bavauto.com
ok, a final question, seeing as the s54 engine is shared with the m3, can most of the tuning products for an e46 m3 transfer over? is the exhaust the same? they r both quad systems, but i guess pipe length isnt the same.
feel free to add