I have a single 2.5" catback into a flow40, exiting out the stock location, and it sounds like an ANIMAL! not sure if a 3" would benefit or hurt in comparison to a 2.5".... the BIG thing here though is if you're going to spend your money, and if this is a custom job by a muffler shop and not a pre-fab kit from Flowmaster of Gibson or whoever, than do your best to find a shop capable of mandrel bent piping, not crush bent.
That is my only complaint with my current system, as the crush bent piping distorts the sound quailty at high RPMs. not sure if it affetcs the power, but I'm sure my truck could be much more of a "screamer" if I had mandrel bent pipes.

 

with no muffler your truck would shatter windows 100 miles away, make babies cry in alabama, pierce the ear drums of the president himself! ok i made that up, but it would be louuuuuud. I'm running a 2.5" through a flow 40 and it's loud enough to set off car alarms just driving by. a flow 50 through a 3" pipe I don't think would get any noticable difference in performance or sound.

 

One thing I have heard is that if its to free you will gain high rpm hp but lose low end torque. Thatswhy many systems make alot of power but only in the upper rpms. Its for this reason that I am looking at the Gibson or Magnaflow in a single outlet. The systems are tuned for the application, even the length of the pipe and location of the muffler can change HP/TQ figures. Overall I'll gain more power and torque where I'm operating 90% of the time. Is the difference going to be huge, maybe not, just something to consider.

 

You could just replace the muffler and get rid of the resonators, and see how you like the sound....

 

i have the imco 3 inch single catback on my 05 4.7. the low end grunt from the truck was neck snapping. when the truck got up into the rpms it was still neck snapping but not as notabile. everyone says were did you get the duals. and they are surprised it is only a single.if you go with the duals go 2.5 but with a single 3 inch.