I have a level 10 character that I use as a healbot and summoner. He has 10 levels in priest spells, 16 points in intelligence, and a Summoning focus of 2.
For the last couple of levels and skill points when he summons a shade, it doesn't seem to be getting any stronger.
I am playing on hard if that makes a difference.
This game is tough, but fun. brings me back to the old D&D Gold Box days.