Please do not only consider Extertion as an Assault Skill, making it revocer only 25% AP will end up being nearly useless for Heavys (you might need 3 uses for an extra shot).
Also reducing the cost to 2 would end up in being infinite as long as there are Aliens to kill (2 WP per kill).
Restore 50% AP is still ok, 5 WP or maybe 4 will be ok. If you don’t kill you can’t spam it like crazy. There are still other very strong skills, no need to nerf anything too much. 
@samshell
My main use for Exertion before the patch today was to get a killing spree, mainly in the first turn to make safe room for my soldiers.