Catless downpipes woud be legal in STU since the 135i has two pair (oops, maybe not if it throws a CEL, so catted DPs), also cold air intake, just no software that alters boost.
You might want to consider 18x9 and 18x10 LTW-5 wheels and rolling the fenders to suff 255 and 275 tires in there. With enough negative camber it would probably work.
|