I've been running the following on my '08 WRX for a while now with no noise problems:
Stock GRF manifolds / COP / UP
Invidia catted DP
Custom 76mm midpipe with resonator
I just added an equal-length header to the mix and now the noise on the highway is pissing me off. I don't mind the tone, don't give a rat's ass about 'teh rumble' or lack of. Seems to me the stock muffler is well-designed for cancelling out the strange off-beat noise with stock manifolds but not good at muffling the higher-pitched noise.
Anyone else had this problem? What's the best solution... different chambered muffler or absorption type? I want better flow than the stock muffler and absolute minimum noise.
Funny thing is that my buddy just went from UEL to EL header and his car (with straight-through, 3" core, 6" tipped puppy-launcher) actually got much quieter.