I tried the 9 in my set up & it made power very easily, so much so I had to cut a few coils off.

With minimal top hat the spring has only 5mm preload which gives me 11.2-11.3 f/lb.( also gets a bit stiff to cock towards the end but not excessive )

The number 10 had 47mm preload to make 11.2f/lb

Jon