I think a lot comes down to software and some of the extra items that may or may not be on the cars. I don't think EVO dynoed the cars, they only took the manufacturer's claimed power. Even so, the BBR did do better. A pulley is a pulley. Also most software if it is done competently will produce roughly the same power. Most of the good software for the Mini is all coming in roughly with the same "real world" figures. From the EVO article it is not so clear as too what the cars had on them. BBR's Stage 1 package is vague, since you can get a camshaft, exhaust and a few other goodies with it. It looks like the exhaust came with it, so it ísn't clear whether it had the camshaft or not.
Some of the other cars I think suffer from things such as super heavy 18" rims which will totally knock down some of the performance.
Some of the other cars I think suffer from things such as super heavy 18" rims which will totally knock down some of the performance.