Just wondering what kind of impact this will have on the NPC. They obviously don't sprint but they do use power attacks. Is that the only thing that is affected when their stamina is low?

Yep. I was actually disappointed that a drain stamina poison wouldn't knock them down once the numbers tried to go less than 0 like in Oblivion. That and the fact that unarmed damaged stamina in addition to health in Oblivion made a H2H fighter semi-usable. Now you have to min-max smithing and use Daedric Gauntlets to even stand a chance.

power attacks only need 1 stamina point to initiate so ;p. yeah Stamina does nothing for npc's they cans till run, still fight with the best of them and all that jazz you'd think would be different this time around.

Only time its useful is if you also poison them with the damage stamina regen effect, which will prohibit them regaining any kind of stamina for a time.