I had a bung on my skunk2 and it made it almost as quiet as an oem system.
Think my neighbours used to get pissed off when I let my car warm up in the winter in the small hours until I fitted my bung
my mate has one and it has quitened it right down, still get awesome sound in vtec but much quiter in normal driving, he got the bung for the same reasons as you travelling long distances
After you posted this, I took mine out to actually tell the difference and it is quite significant, although as I said on CL, its still loud in the 3-4k rev range and when you're doing motorway driving, it can give a bit of a headache!