Let's say that hedge wizard warcasters get the Adept spell list.
6th level warcaster, 20 Int, has built herself a shrine. Wants to cast 1st level spells that aren't on her list.
+6 ranks, +3 class, +5 Int, +2 shrine bonus. That's a +16. (No synergy bonus from Arcana--if she were in a position to pick up Knowledge: ARcana, he or she'd be a book-wizard. OTOH, she's been adventuring or whatever for 6 levels. So she's picked up 6 ranks in Arcana, without the class bonus.)
MAke that +6 ranks, +3 class, +5 Int, +2 sanctum bonus, +2 synergy. That's a +18. Add +2 for the Magical Aptitude feat, +3 for Skill Focus (Spellcraft) and you're at +23.
I'd say that character build should be able to spend d6 HP to cast off-list more often than not. DC 26 means, in this case, a 10% failure chance. That seems fair.
The 2nd level elite warcaster with the 20% chance of success? You could argue that just having the +4 stat bonus makes him a prodigy. So, if he's willing to burn d6 hit points with a 4-in-5 chance of complete failure, maybe that's okay.