There's another thread on these boards (in this Forum, I think) that discusses a KoK feat that changes the save to "None" (for a +4 increase). The general consensus in that thread is that the feat is broken "at any speed" (the discussion went as high as +8).
What you're proposing closes one of the major safety valves in spell design. Consider what would happen if you applied this to Hold Person, or something similar.
My gut feeling is that it should be at least +9, and +10 sounds about right.