Just to add to the speed loss numbers. Heres some info I came up with, while experimenting. I found it interesting.

In a trimmed biscuit with notches where the vanes pass through the notches - no speed loss with a 3 degree offset vane, gained 1 fps in one setup.

In a trimmed biscuit (no notches), a helical vane lost 2 fps over an 3 degree offset straight vane.