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
ESS VT3-750 - 60-130MPH 6.14s - 10.81 @ 135.13 MPH 690Whp/463Wtq
Shift-S3ctor E92 M3 - 1/2 Mile Trap Speed WR - 174.13 MPH