Cheers Brillo. Yeah, I did this, although I didn't have an OEM back box, mine was the Japspeed K2 (so very similar in design), it didn't make the difference I was after. The OEM mid-pipe is 2.25 inch in diameter and I replaced it with a 3 inch diameter straight through pipe (no mid-section silencer). It DID give a nicer, deeper sound, but if you want more volume (i.e. an exhaust that turns heads as you're approaching), it's not worth trying to acheive it this way. Put the money towards something else (I bought a K1 shortly after trying my mid-pipe experiment).
The actual volume/sound is governed a lot by the construction and design of the back box. As the OEM back box (and K2) is a cambered design, the 'sound' has to face a lot of obstruction before it reaches daylight. A performance back box (such as the Cobra), is less restrictive, allowing the gases and sound to travel more freely.