I'm currently running a set on my 81' Invader, I don't have any dyno numbers but I can tell you that my butt dyno says that it's faster. I suspect it would be very similar to the power a stock 4/6 makes.
The 81 with the kit pipes and Polaris clutches is noticeably faster than the 4/6 with Aaen pipes I had a few years back. This had more to do with the clutches than the pipes though. The Aaen pipes had an extremely narrow power band, that in combination with the stock Kawi clutches and their known issues made it nearly impossible to tune.