Skill and +macc should be just about equal. It's possible that skill follows the pattern of combat skills, in that skill past 200 is only .9 accuracy, though.
As far as I know, it's always just been extrapolated that int has an impact on additional effect accuracy, but I don't think I've ever seen any sort of real confirmation of it. (Actually, am I remembering a test someone did rdm/blu with headbutt with varying int levels? That sounds familiar, but it's been like 400 years.)
Anyway, for elemental magic, int is supposed to be equivalent to 1 macc until your dINT is over 10, then it's only .5 acc. It's reasonable to make the same connection for additional effects.
And now I'm rambling, but thinking further on it, where spells like vapor spray and barbed crescent are totally ****** up with their added effects, it's possible that different spells all have different junk that affects their added effects' accuracy rates. So, I guess I'm not particularly helpful at all here!
tl;dr: I wouldn't bother with going out of your way for anything except a staff, skill, macc and int or haste/recast in other slots that you happen to already have.
Louverance: You should spend your next life as a friar, my friend! Later...
Meransarget: Oh, herro there.