OEM exhaust failed in abig way a few weeks back - went for the Scorpion RS192, seems to be fine - MUCH cheaper than an OEM replacement, plus lighter of course. Not too loud, not too chavvy, just a nice deep note.
any performance gains (5bhp max?) would surely be offset against the gained weight that dual back box designs add over a single back box design, so it's horses for coarses.....