I've never had a BXR manifold, so this is all just rumour and speculation.......
Tim Ellis @ DynoTech detailed a comparison on his website about how bad the BXR was compared to a GT40 manifold. He had installation problems, machining problems, and it made less power and MUCH less torque than the GT40. BXR sued him and had his website shut down, claiming that the test was unfair. Tim Ellis was a well respected participant at
www.corral.net with many people going to him for dyno work, turbo/blower installs, engine builds etc. He was unable to elaborate further on his problems with BXR, but you got the idea that they just didn't like bad publicity. Since then, I've seen a couple of dyno tests where the results certainly didn't justify the outlay. No, you won't LOSE power when compared to a stock manifold, but you'll be sacrificing potential power that you should get with a TFS/Holley/GT40/Edelbrock.