Neruse

Aye, there's a good chance it'll be worth pulling points from Aspect Mastery for Longevity. Hopefully pets scale well enough.

Taking another look at Kill Shot:

Level 80
475 Mana
Instant cast
6 sec cooldown
You attempt to finish the wounded target off, firing a long range attack dealing weapon damage plus [RAP * 0.15 + 325]. Causes an additional 745 to 825 bonus damage against targets at or under 20% health.

Extrapolating from the gain from 60 to 70 high-end, raid buffed hunters, I'm going to assume 5000 RAP, 1750 average weapon damage.

1750 + [5000 * 0.15 + 325] = 2825, 3570-3650 under 20% (non-crit).

Damage gained is significant, at the price of increased mana use. If hunter regen is strong enough it may be possible to weave KS in pre-20%.