First of all, you can use as much helical as you want, especially with feathers. I found that even 4" full helical vanes only cost 2 fps compared to straight offset vanes. With feathers, there should be no speed loss.

Though obviously you are shooting very well, I suspect the rest is off just a bit. The logical move would be to move the rest lower, and see if it helps, then maybe left a bit in separate adjustments. The Biscuit responds to very small changes, so only move it 1//16" or less at a time. The other thing to keep in mind is that the Biscuit can also show "reaction" as well as "action." The arrow could be coming off slightly tail high/right, and what you are seeing in the BHs is the reaction of the fletch end of the arrow being forced down and left by the Biscuit. I've often solved Biscuit tuning issues by moving in the opposite direction from what would seem logical. I'm guessing you are shooting the 400 size arrows. I think there could be a spine issue @ 70# with a fast bow like the Switchback, even with 27" arrows.

Just out of curiosity, how did the bow shoot with the Muzzy? Did you shoot broadheads with it at all?