I found on the cobb website:
We have found that the 135i stock catback exhaust system is restrictive compared to other BMW models. To achieve estimated HP and TQ gains as listed in the map notes, an upgraded catback exhaust with less restriction is necessary. Retaining the stock catback exhaust will result in lower than estimated HP and TQ gains, but will not adversely affect the safety and driveability of the tune.
I`ve already own a cobb AP and just ordered the BMW Performance Exhaust.
As the new exhaust replaces the axle back exhaust AND the absorber in the midpipe I just wondered which part is the restrictive one?
I also planned to change the downpipes with 3" catless, would be interessting if I can eliminate the restriction by just changing the axleback exhaust without removing the absorber in the midpipe.