The manual system is one of the more aggressive systems on the market. It's just a couple of resonated style mufflers so once they break in I think you would be very happy. Another option to add some volume would be a set of high flow cats. Teamed up with the MBRP catback you would get one killer rumble without adding headers
The MBRP catback is pretty aggressive. I is probably a bit louder then your Flowmaster system but if you want a real loud set up you'll need to get rid of the factory manifolds and cats. You'd be better off spending the cash on a set of long tube headers that will not only give you the volume you're looking for but also the horsepower!