Unless your putting out a serous amount of power, I doubt the flows will cost you much if any at all. As for the drone, I have the 30 series cat back(3 chamber) and there is hardly any drone, just a tad around 2200 rpm.
I also vote for the o/r-H, Im putting on BBK LT's and o/r-H later this summer, so I will tell you how that goes, and try to get a sound clip if anyone wants to hear it.
Also, the 30 series dont sound quite the same as the 40 series does, its alittle deeper with less drone and a bit quieter. They still have the great muscle car sound, but with alittle more rumble to it. Not sure how to put it exactly.
2008 Alloy Metallic Mustang GT Premium
5 spd manual, Interior Upgrade pkg, Premier Trim pkg, spolier delete
LT headers w/ O/R-H, JLT-II CAI, Bama tune, CMDP