I've actually found this interesting, as the GCG isn't as responsive as the stock turbo, so wouldn't be making pressure at the same rate, how would it have more power in the 2500-3000rpm range?
As mentioned before, I'd understand from idle to 2500rpm purely from the better flow of the exhaust, but this wouldn't account for any extra power in the rev range where the stock turbo is already at a decent shaft speed and the highflow just wouldn't be at the equivalent speed...there must be at least 500rpm where the GCG doesn't make more?