My Googlefu shows they are a copy of the flowmaster 40 series, that would be significantly louder than oem, but it would give you a decent idea of what it would sound like. Lots of folks here like borla and mrbp, have you looked into those?
It wouldn't surprise me if the gibson flowed better, our stock mufflers are massive. I can't comment on the sound level from the gibson, have you tried searching youtube for a sound clip?
Edit: I meant thunderbolt muffler not gibson, it's been a long day
Since the wifes car died unexpectedly, I'm kicking around the idea of giving her Shelly and getting myself a 4wd ranger, I'm really digging this one but in the description it says off road screen, what is it and is it just a gimmick...
I'll post pictures tomorrow but Shelly and I just did a 400 mile round trip to recover the wifes broke down car. I'm super impressed with Shelly, most of the time I couldn't even tell I was towing a 3500lbs car
I'm curious how much the additional weight in fuel would hurt gas mileage but I'm definitely putting this on my list, call it a minimum of 800 miles on a tank of fuel would be fantastic