1) the stock 2.25" mandrel bent exhaust flows well enough to support ~400HP without significant HP loss.
2) as exhaust cools, due to the ideal gas law (PV=nRT), it's volume reduces (for a constant pressure, P, as temperature T goes down, volume V goes down), so the resonators way in the back where the exhaust is much cooler than it is at the manifolds really aren't much of a restriction.
3) as far as drone is concerned, one of the big factors for resonance RPM is pipe length, especially free length after the muffler. think of a pipe organ--longer, larger pipes have a lower resonant frequency...or think of blowing across the open top of a 2liter pop bottle---empty, it's a low tone...fill it half full with water, the frequency raises substantially....to minimize drone, you should put the mufflers as far back as possible, with short tailpipes....which is basically what the stock resonators are...mufflers all the way back with short tailpipes.
'11 Toxic Orange Charger R/T
'06 Dodge Grand Caravan
'76 Plymouth Duster