I'm not sure about raw power gains from BCS system, but based on my limited research/understanding and a long chat with Nige from BCS (while my system was being fitted), it's one of the least restrictive systems available, with full 3" from turbo. Additionally, with the powervalve option, you get the benefit of more torque in the lower rpm range due to the extra backpressure caused by the closed exhaust valve when off boost. Then there's the very high quality build of these systems... Definitely hard to beat even if you factor in the price for a full system.
In any case, I guess that it's always about the overall set of modifications that have to complement each other, so a BCS system alone would not necessarily give more power than, say, a Miltek. Would be very interesting to see how both compare when paired with a good CAI, map and uprated HPFP! I wonder if anyone else is going to the April's AMD RR day with the above mods, but with a full Miltek system? Would be great to see the two side-by-side!