I run into people out riding that have those loud exhaust on RZR's and 4 wheelers and find them quite annoying myself. Now a loud V-8 is music to my ears, but anything less than 8 cylinders just sounds like crap IMO. I really despise hearing those tuner guys in their 4 cylinder bumble bee sounding cars.
My snowmobile riding buddy was telling me awhile back about a friend of his who got one the newer Ford Raptors that have the Eco-Boost V-6. He put an exhaust system on it and he said it just sounds horrible. Even the guy who owns the truck regrets it now. Sorry got a little off topic there...
I've used these exact muffler's on 2 different 800's. In my experience I didn't feel like they were excessively louder than stock. SLP says this is within 2 decibel's of stock. Like Darth said if noise is an issue for you go with the Super Silent Muffler that is 5 db quieter than stock. And yes there is a small improvement in horsepower.
FYI... SLP says this exhaust is 90 dBA using SAE J1287 test. 96 and above is a fail and will get you a ticket in Moab.