Deep thoughts, by Jack Handy...
The fiberglass packing in the muffler, or resonator, does not add resistance per se or affect backpressure. All it does is dampen the reflecting sound to help reduce the resonance and blend the exhaust note. Extra baffling in the muffler and smaller pipes will create back pressure.
Theoretically we should get some minor improvement in performance with higher flowing exhaust through the scavenging effect it is supposed to make. Meaning it will help "suck" the exhaust out of the piston chamber as opposed to letting it be pushed out. I guess this is supposed to reduce the effort the piston makes on its second rise to exhaust the cylinder - in essence improving efficiency.
Were the duals and high flow cats and open mufflers will shine is if you improve intake air velocity/amount. i.e. better intake, and ultimately, a supercharger.
I'd say the more insulation the better as opposed to constraining flow with baffles (my old Morgan tec had no insulation and basic baffling so it liked to develop a nasty resonance at a specific rpm due to harmonics - insulation dampens this effect). I think Magnaflow mufflers are on the loud side due to their goal to maintain high flows with minimal baffling. From my experience, they should be complimented with a resonator or glass pack to tone em down a bit.
Personally, I'd lean more to a heavier muffler just to get the sound levels down. If you don't plan on supercharging, and want a growl, I think one of the heavier models by Flowmaster, Cherry Bomb, Aeroexhaust, should work.
I's say our biggest challenges are being 6-cyl (we all really want that 8-cyl purr) as well as a very short underbody with little room to play with pipes and such.
