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I came from a Subaru as well, but I was running a 4.5 inch turboback exhaust on my car considering I was running a huge turbo.
For the 135i, the midpipe size, 2.5 / 2.75 is more then efficient when it come to power. Since our cars run a single outlet exhaust, a dual midpipe is pretty much useless.
The main power source is our downpipes. Our oem downpipes aren't free flowing at all, plus they weigh a ton.
If you want to see some results, swap out the downpipe. For our cars, the n55 runs strong with a 3.5inch downpipe or 4inch. Hope this helps.
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