RE: Difference between 8t or 9t pinions
It also depends on whether you have plasti-blades or not. I have a post that discusses this.
My BCP would not fly with an 8T and plasti-blades. It's fine with a 10T (but I'm worried about over-stressing the 4-in-1). I don't have a 9T, and so I have no data there.