If there was a rear section + tune that could produce 40RWHP we would all have it. This guy is 100% wrong on everything. Full catless exhausts from reputable companies like Dinan and Akrapovic only claim 25HP with a tune. Check Dinan's site for their "white paper" article on backpressure. It's a good read.
2009 Silverstone II E92 M3 Coupe - DCT
Dinan Exhaust, RPi scoops, Dinan Tune, Dinan Pulley, BMC filter, LCI Taillights, MS 15mm spacers, Huper Optik tint.