Just thought Id mention in relation to this discussion, I went and bought the Xforce system today, just the cat back section on its own, and also the Miltek catless front pipe to suit the twinscroll JDM, from Redline today. They ended up both mating to each other perfect, so that was a win. The only thing is the price is a bit up there, was $1590 for both, but happy with the fitment, perfect right the way through, only issue is the rear o2 bung is further away. So just extended the wires to the sensor and otherwise no hassles. Also both parts came with all bolts and gaskets to complete install.
Pretty quiet too when driving normally too, barely audible at 100kmh cruising. One thing I noticed though, is the GRB's sound a bit different to the GDB models when on song. Sound a bit raspier like the twin scroll Leggys do with an exhaust. Anyone else with a V11 noticed this ? The headers look slightly different design to the GDB, similar to the Legacys so that and the dual AVCS heads must give it the different sound I guess.