I haven't seen a specific post on how much INT is affecting spells but I haven't really looked either. Just by looking at figures that Kalabus (50 Warlock of Ascendance) posted in his screenshot / feedback thread of how it currently is on test:
Going from 235 to 267 INT (his first post in thread was 267 and one with screenshots 235) a rough difference of 3.2% for 32 extra INT. I can't verify but it appears to be with each 10 INT you get 1% increase in spell dmg. This was taken from his word description of Nil Distortion and then the screenshot of it in his posts at each INT amount.