The OBX system with stock GT mufflers is NOT loud at all. In fact it's very stockish sounding. Now with the GXP mufflers it would be much louder than stock and a great option if you don't mind how loud it gets when you get on it. The Solo axle backs paired with the OBX is just too loud, you need to add a muffler or mufflers of some kind. The GXP mufflers are hard to find, it's why I mentioned the Dynomax bullets and the fact that everyone who listened to them liked that set up better than the sound of the GXP mufflers.
I'll have to try and make a video. Pacesetters (connection pipe flange taper cut off, so still 3") to 3" X pipe and 3" Dynomax bullets (I think 21" or so long) with 3" pipe to just before the axle back where it tapers to 2.5" (this is very much like what the OBX system does) then Solo axle backs modifed by adding the short dynomax bullets.
My set up is about the same volume as the GXP mufflers, just much deeper. This is why I know the stock mufflers are very quiet with the OBX, and the GXP mufflers are much louder than GT mufflers. Also the Yeti has the OBX and runs stock mufflers some times.
I have done an extensive amount of different exhaust set ups on the G8.......too many!