Magic Resistance mastery for Aegis makes sense.
(in fact, that's pretty cool 
)
Here is a list of mastery options
https://forum.atlas-games.com/t/compilation-of-spell-mastery-options/3019/1
Tellus, you're using a level 20 spell for your penetration computations? Is that already locked in for some reason? (It seems like a wise choice but I don't see where it would have come from.)