Oh it's gonna be loud alright! Especially if you have one of them exhaust cutout switches! I've heard my 5m uncapped at the collector, and thought I was gonna shake the tools off the pegboard! Even with a cat and Magnaflow, it's somewhat loud, but it does fall .6db short of being illegal in California! :?
Remove the cat and gain an extra 25db at around 4500rpm, but gain a few hp at it too...