I haven't messed with the exhaust on my 01.. but I did have a vance & hines system on a 750 katana a few years back.
It worked well.. very .. uhh.. "racy" sounding to say the least.. it made that 750 rock. It always got compliments .. especially at 10,000+rpms very SCREAM.. very little Grunt.
I never dyno'd.. so I dunno about HP gains. i'm sure there had to be some just from how much freer the air could move thru the system. Very easy to install.. looked good. worked good, and lasted well.. had it for 4 years.