Quieter than with LTH (which I am currently running)
Edit: I have yet to run a dB test inside my cabin, although I have the tools to do it, but since I've run a couple setups, I might as well actually say something useful here.
The FI exhaust non-resonated with stock cats is actually very quiet. If you've heard a Motordyne with stock cats, it is MUCH quieter than that. Adding LTH, the sound becomes... epic is probably the right word... but the drone becomes much more pronounced between 2200 and 2800 rpm. I eventually swapped over to 12" resonated midpipes when running with LTH because of the drone, and that was a decent fix.
I've never run with test pipes on this setup (got this set from a guy parting out) but if you're not using this as a DD with a lot of commute time @ 2300 rpms (which I do), it will be not only manageable but very enjoyable. FI has a really nice low rumble at low RPMs and absolutely roars at high RPMs. Plus it's hard to argue with their hp gains. YMMV. If you're concerned about volume, I recommend an HKS.