It depends on the meth setup. AA's kit is more like a charge cooler to keep the IATs down. I don't think AA changes their tune for the meth.
Something like HPF's meth kit on the E46 is a power adder and the tune accounts for it. That is also a somewhat complex system that has a lot of safety nets. From what I know, it is a reliable system as well. My want to look there, look at their dyno charts between the meth and non-meth systems. Might give you an idea on how much power you can get.
IMO, I don't think you'll get good/safe results unless you have a REALLY knowledgable and good BMW tuner nearby and have lots of money to throw at it.
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