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In decreasing order of importance, the best ways to get easy power on a N55 are:
MHD tune (custom dyno remap optional)
catless downpipe
intercooler / water meth
e85 mix, but really if you have to mix gas every time you fill the car it's probably time for a turbo upgrade..see below.
The stock air filter is pretty good and replacing it will make nearly no difference on a stock turbo and you will lose a lot of filtration which is bad for the engine.
There is also a restriction in the charge piping and probably a muffler on the turbo inlet, but again, diminishing returns. The stock turbo is very small and you need to replace it to get a "serious" power increase. Pure Stage 2 is the easiest way to do that.
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