This may have changed but once upon a time I heard it was beneficial to use +magery items OVER your cap to help get 3 mod books. I honestly don't know how legitimate this was but I heard it from several sources.
I suppose the Max would look like this
120 Real and Displayed Skill (as anything over 120 won't display of course, or even less if you haven't eaten a 120 scroll)
+20 Crystalline Ring
+15 Bracelet
+5 Talisman
+10 Mark of the Travesty
Not sure if I missed anything here, but that is sort of what I recall.
To answer your original question, Having 120 Magery with no +skill items should give you the same bonus as 120 Magery achieved with +skill items.