it depends on the psi gain its actually provided you with, there isn't gonna be a set figure of power because there are so many variables, but decent free flowing exhausts generally give upto around 5hp, maybe slightly more, but pave the way for further exhaust mods, decats and exhaust manifolds etc. the only way you'd know for sure is by comparing before and after printouts from a dyno. didnt mean to sound rude mate, just stated the obvious that if you're hitting fuel cut after having an exhaust fitted, then its obviously more free flowing than the stock item. i have a cat back tanabe stainless exhaust system, its very rare, but won't come into its own until i invest in a decat and/or exhaust manifold. i did hit fuel cut once or twice before my clutch started slipping, but havent since obviously, once its done i'll see how close i come, then invest in the decat/manifold and a boost controller so i can set the boost to peak just below fuel cut, saves keeping an eye on it then.