Yes, when using a targeted skill like stasis you can use the small arrows by the hit percentage to popout more details, this will show all the boosts and drains to the hit%. IIR will is directly used as the base hit% and has the enemies will subtracted from it.
I believe the aliens get the same formula this could be check by mind-controlling a sectiod while one of your agents has been controled.
Edit.
@ChrisHayes and @DarthPizza have corrected my bad memory in that psi attacks use the 'Psi Offense' stat to hit and 'Will' to defend. I'll have to check if any of my high will agents have an increased psi attack stat but I don't think they will