We have tried them on several of our kits and no longer use them. We install the primo pro clutch and forget about clutch problems. Have the bike dynoed again and then you will know if you have slippage. Seat of the pants will not tell you the whole story. If you feel it slipping then it is bad, we where getting 2-3% slippage with the monster VPC, with ramps, heavy spring. And by that time the pull was heavy on the lever. The primo is more money but not by much after you use the previous parts, the lever pull on the Primo is very light and it will not slip. Same clutch pack is used in the open systems. I make 159/163 and there is no slipping. At the track I leave at 3800 in 2nd gear no issues. I have adjsted it a few times over the last two years. It does not wear like a stocker. A VPC works with RPM so you are still working with the small amount of disc surface, and cheap discs from HD. Not the deal with a Primo.
I have other customers who like them but one thing you have to look at is how the engine makes power, as you have many items that can effect how well it will work. They are working with rather mild builds.
We have plenty of customers try the heavy disc first them they will go to the VPC in the end I get a call as they are done messing with it and buy the primo. This is just my opinion , it is based on the amount of builds we do. Not bashing the VPC but I do not think it is the end all be all. I have removed two many of them for slippage, some was after getting a few thousands miles on the set up. Riding style is going to play a role here as well.