so, turbo inlet pipes are required to carry HP past 4.5k rpms? not a ported head or upgraded turbo's. honestly, in my opinion, this shows the strengths (and weaknesses) of the RB turbo's. they're midrange monsters. upper RPM's though, same story as it has been for the past few years. the turbo housings are too restrictive at upper RPM's. power falls off, EGT's probably are higher, etc.
for me, if i was going to spend thousands of dollars on my e92 for a ported head, RB turbo's, install, etc......i'd just the same get a single turbo kit. that power band is better suited for drag racing and 60-130's.
i will say that RB turbo's with their massive midrange power probably make for an excellent street car.