What exactly do you mean by slow? Like, the car doesn't want to rev up at all or there seems to be a delay between pedal input and engine response? The 06 should still be cable driven throttle body, not drive by wire, so in that regard at least it should be fairly instant. Wondering if maybe the problem is somewhere else like a clogged cat or something restricting the engine otherwise. It's also a large displacement V10, so I guess it depends on what your definition of "fast" is.
Yeah, that's pretty common of these cars. The short exhaust and side exit makes it more obvious to the ear as well. Typically that's just some unburnt fuel in the pipes. Some people really like the sound and will even tune to have more of it.