extractors, single 2.5, highflow cat, one good muffler... this is a proven all round setup, still retains great performance all thru the rev range and can sound great with a quality muffler. had this on my VN and made plenty of power
twin 2.5 gives you scope for more mods in the future. Probably not the most ideal performance wise as a stock 304 can lose a bit of bottom end grunt. But they sound great. Biggest hurdle with a full twin though is the price, as you are pretty much paying for x2 cat back systems. can get expensive for pretty much just a sound (on a stocker that is).
Single 3 inch is a good cheapish option as you dont have to change cats/ pipe configuration. The shop will simply plumb a new pipe from the cat back. and with a good muffler will sound great!
twin 3 inch is way overkill if you're not going major internals or FI. would probably sound pretty killer, but i'd say it would go like a slug.
personally, i went for a twin 2.5, twin cats, into single 3 inch with one muffler in the middle. Sound way tougher and deeper than the single 2.5, yet maintains similar performance as opposed to before. Prob just a fraction slugish down low. I did this change purely for the sound, and cause the replacement system was only $120 second hand in vgc
