Best way to see if there's any real gains are Vbox runs, 1/4 mile runs, or do a comparo with a bonestock car before and after the exhaust. If Turner Motorsport is correct, the real gains come from software and front cat delete, gears as well, but muffler's I would think not much.
08 E92 M3 DCT - Bolt Ons - 60-130MPH 10.71s - 11.88 @ 118MPH - 377WHP
ESS VT2-625 SC 60-130MPH 6.80s - 11.30 @ 129.3 MPH 586WHP / 379WTQ @ 7.2psi - 94 octane / Water meth
ESS VT3-725 60-130MPH 6.64s - 620Whp/420Wtq@ 10.5psi
ESS VT3-750 - 685Whp/466Wtq@13.5 psi SAE - 703WHP /478WTQ@13.5 psi STD