Without a muffler of some sort, yes, it would sound terrible. When we did the EJ22 conversion, he didn't get the exhaust done immeadiately so it ended right after the cat. Boy, was that LOUD! It sounded terrible too. I heard a EA71 with straight pipes - that sounded mean, but way too loud for street driving.
However, if I use a set of glasspacks or turbo mufflers, that should keep the noise at a reasonable level, but still sound mean with duals.
I toyed with doing duals on the EA82 in my old 86 wagon, but decided against it due to lack of low rpm torque. Since the new engine will be 2.5L, that's 1250cc per side. Should be able to find a good pipe size with that. Anybody know what the size of the EJ22 y-pipe is? I know it's double walled to keep the noise down.
That's option A, option B would be get a set of equal length headers, resonator, and an STi muffler to put on there. Quiet, but deadly.
The other thing with true duals is whether the ECU will be able to compensate for the fact that it's only seeing half the exhaust gasses....If it goes nuts, then I will have to use option B. Choices choices.....
A local exhaust experimented with their 87 carb wagon during the early 90s (they had a dyno as well).
They found that dual 2.25" pipes to just in front of the rear diff gave the best compromise.
How this would relate to the later larger EFI motors is anyone's guess though.