what exhausts out there would give me the most gain in HP for my 06 530xi??? got a couple in mind but cant decide.

I would go with RPI.....sounds nice and you can install it yourself!

I can speak for my experience of having the RPI exhaust and I am very happy with it. Sounds good on the I6 and fairly comparable to the V8 engine exhuast note when compared (I heard my buddy's 545i with RPI exhuast side by side - could be wrong, but just my opinion).

Note that you will experience cabin drone however. This can be somewhat mitigated by installing a silencer that RPI produces.

Good luck with your decision.

What if I convert it to a dual exhaust.....? Lose power or still gain? If gain what should I go with?