I have a catless perrin downpipe, 3" exhaust shop custom, in to a prodrive muffler. the sound of power is discovered, not announced. favorite thing to say about my car is that it doesn't sound like a stupid subaru. lol
I don't know how restrictive a stock GD WRX muffler is, probably more than less, but I loved the sound of the one I had. It rumbled nicely but had no rasp and was definitely on the quiet side. Honestly I wished it was a little louder, but you could hear it.
I have headers, high flow cat and 2.5" exhaust (this is with a supercharger, not a turbo) and it wasn't until I added 2 Magnaflow mufflers and a finally a resonator that it was tolerable. The resonator made the biggest difference.
As many resonators as can fit, plus be sure to have the largest muffler possible. I had one with the larger oem style muffler but the tip was 4". It was better than the cannon muffler style but still fairly loud. Got the invidia Q300 and it was a little better because better muffler baffling.
This time I'm getting several resonators and a large Apexi universal muffler, all done together. It will still be 3" minimum pipe all the way back, so long as the resonators and mufflers don't reduce diameter it shouldn't affect power enough to notice.
Resonators are pretty much tiny mufflers, so they just mellow the tone even more before it gets to the muffler. There is limited space under compact vehicles, so anywhere they can cram additional muffling is advantageous.
I've got a FEVER, and the only thing that can cure it is more WHP!!!