Hi Mark,
Mine's the same - it's pretty noticable at around 4-5000 rpm (in other words, right in the middle of the famed v-twin mid-range), particularly with a pillion, where the bike is under more load.
I've actually revert back to std cans with a power commander map for those, and it's better than with the race cans.
When i get back from holiday I'll get it on a dyno.
cheers,
Nik