I'm new to the WRX so take my advice lightly but I drove one MY00 STi WRX with a silencer in it and it absolutely ruined the car, worse than stock. My MY99 WRX was much faster. Some silencers are more restrictive than others. They do however make a big difference.

If it's too loud you could change out the muffler, cannons are the worst if you have one. You could also check the resonator to reduce drone, but won't affect overall noise I don't imagine.

equal length headers won't affect drone or sound level, you'll just loose the Subi rumble.

I get up at 2:30am, one neighbour used to complain about my stock exhaust. They moved before I put in my Hitech :P
Just leave the area quick so your neighbours can get back to sleep :P