the NA version of Apexi N1 for 2.5RS isn't really loud on 2.5L 5MT but louder on 2.5L AT and 2.2L, had it on my sister's for couple years ~ yet i prefer to run it with the silencer around the city, better response but definitely no silencer on the hwy (smoother and yet can hear some nice exhaust note)...
yet for universal muffler wise (since u said a muffler welded onto stock exhaust), it might be a diff story (a friend had a small canister one ~ NA version and it turns out to be louder than another friend's with a bigger canister ~ turbo version)