im very impressed with the PSD system fitted on the g40, obviously they no longer exist, but PPP sell exhaust systems made specifically for the g40 and they look fantastic, im sure the quality is top notch, as they are made by a company with very good motorsport credentials.
pricey though, but any custom system would cost similar amounts from your longlife/powerflow dealer and with them tried and tested on Gs you cant really go wrong
they are in the the sponsored section of the forum and im not affiliated but appreciate quality