How good is the stock exhaust on the 135?

I'll venture to say it is really restrictive. I'm assuming it has at least 3 cats. It is single exit compared to 335's dual. w/o a tune a 335 can achieve a gain of 26whp - 40ft-lbs/tq with aftermarket downpipes. I think it's safe to assume 135's will gain at least that through aftermarket avenues.

But it has a great sound! :)

I'll venture to say it is really restrictive. I'm assuming it has at least 3 cats. It is single exit compared to 335's dual. w/o a tune a 335 can achieve a gain of 26whp - 40ft-lbs/tq with aftermarket downpipes. I think it's safe to assume 135's will gain at least that through aftermarket avenues.

But it has a great sound! :)

Who is making aftermarket downpipes/exhaust for the 335i's currently?

uhh ripp, borla, britalman, eisenmann, etc.