When I have observed Circle C rolls vs. factory rolls in otherwise identical headers running in the field it always appears that the capacity is reduced considerably due to poor feeding. The rolls just aren't as aggressive as intermeshing rolls. There have been many iterations of groove patterns over the years in an attempt to solve this issue. My experience is from quite a while back so maybe this has been improved with later designs. A number of the operators we interact with used to have one or two of their machines set up with them but now it seems you just don't see them around anymore, at least in the areas we work in. The factory-made conditioner designs have improved over the years and as a result the benefits of aftermarket systems has decreased, especially when warranty and resale value is factored in.
This is just my experience from field observations and talking to windrower owners and operators so take it for what it's worth.