There's finally a direct-fit aluminum radiator by Mishimoto available (for 6MT only)! It hasn't been out for more than a couple months, but has anyone tried it? From all the feedback on these forums, it appears improving water cooling is more effective than oil cooling for preventing limp mode on the track. I've been considering adding just the auxiliary radiator from the PPK2 but this could be the way to go. Or both!
http://www.e90post.com/forums/showthread.php?t=950469
http://www.e90post.com/forums/showthread.php?t=999461