As one increases the action point regeneration and the other restores action points entirely if you kill someone in VATS?

No, no perks are mutually exclusive. If you DON'T kill someone in VATS, then Action Boy will help you regenerate AP faster... if you DO kill someone in VATS then GR's sprint kicks in.

Yeah. Maxed out, Grim Reaper is only a 45% chance. So if there's 10 enemies in view at the same time, and you wanna mow em all down, you need to kill on average 2 enemies per full action point bar to keep on shooting. Hard to do and randomness means sometimes you won't get the sprint even after making 2 kills.

You probably want to have power armor on if this ever happens and 2 points in action boy, so when you have to "rest" outside of vats you can regenerate as fast as possible.

It's also a nice perk to have because it means you can spend more time sprinting when doing overland exploration.