I'm using the HKS exhaust with no cat and I find it not too loud to drive (since HKS exhaust itself is pretty quiet) i think even Gruppe M's ARC exhaust is louder than mine even he doesn't remove the cat. However, another friend of mine using the RSR dual exhaust without the cat, man, his car is LOUD! but driveable of course!
for comptech exhaust, i think it should be as loud as the RSR one and the sound is sweet~