Downsize the pipes into the muff? Come on man, that's just bad form... :P On my old V6 I did a 2-1 Spintech, and sans the resonator (that tiny can at the rear next to the spare tire) it was too loud, but with it on it was just perfect. But, this is all subjective. Some people like the Gibson Cat-Bac...
My testing of the Powercore filter showed it to be similar though somewhat less restrictive than stock, while the cotton filter was extremely unrestricted compared to stock.
I'm more a fan of the SpinTech mufflers myself... a nice 2-into-1 to replace the primary center muffler should be easily done. If you're more looking for in-cabin sound instead of annoying the neighbors, I *love* what the Volant Powercore intake does for it. Great Merc AMG V8 like growl from under t...