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BPM for sure. I have done it and it makes a huge difference. The test pipes sound great with whatever exhaust you have. I had a full borla system, with the test pipes, and against a stock M3 it is much faster. If you have DCT make sure to get their DCT upgrade as well. I have it, and the car LEAPS forward with each shift. If you have manual, it also has a launch control, that a buddy of mine has, and it is awesome as well. Also has a cold start rev limiter, and you can raise the redline to 8400 or 8500 as well. You also have a few great guys, that can get it done quick and really care. I think the BPM tune is the biggest bang for your buck, and you wouldn't have any regrets.
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