I've replaced the stock system on my H2 with the Akrapovic full system sold by Kawasaki as an accessory. They had an offer on them a while back and charged me £1500 including a new muffler, which is bigger bore than the Akra' supplied with the bike.
I looked at the Vandemon and Brock's Performance but at the time there were no noise figures available for them. We have a 103dB upper noise limit at Straightliners, over that and you won't be allowed to run. With it's baffle out the Akra' usually measures around 98dB or below at the 4,000 rpm they use for these tests. It still sounds pretty good, passes the MOT test without any problem and people don't regard you as a nuisance on the road. Of course you could ride about using loads of revs and blipping the throttle, you can do that on any bike, but with the h2 you don't need to.
Go Fast Or Go Home