All I can say is, the best pellet I've found for killing things in a sub FAC airgun are .177 Bisley Mags/H and N Baracuds.
We've got naff-all power to play with and down range performance is vital.
I've been playing with .20 for a while only to see how Dynamic SPC 5s perform. Ballistics is a complicated science and I liked the sound of them, so I thought I'd give them a try.
So far, so good. As you've so rightly said before, it's all about being able to hit the target exactly where you need to, but it's also about knowing that the pellet's going to do what you need when it arrives