Even with just the V6, a single isn't going to get you the same kind of rumble as a dual. Be it true, H, X, Y, etc etc etc, dual is going to have more rumble.
Its kind of like a boombox. You have two speakers with FM stereo, you're going to get better sound than a handheld radio with FM mono and one speaker. (does that make sense?

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Also, for duals, you can either cut the bumper yourself, of just have the muffler shop reroute the tips to pop out just under the bumper. It looks fine.
(EDIT) You can't do that...I forget that there is already a bezel on the v6... my bad...

(double edit) I suppose you could order a painted GT bumper
