To my surprise, i saw _ALOT_ of Spell Deflection-procs on Festergut. I thought it only triggered on "direct spell damage". I did not have any addon to log procs (if you know one, please recommend), but i am curious to know if anyone know or have any experience regarding the uptime. Right now it seems like a really really good talent on magic heavy fights.
I noticed (via Gravity on Twitter) that Acclimation had like 88% uptime on Festergut (for some frost DK called Hive).
There isn't really an "uptime" it's just trigger equal to your parry over each casting/pulse of the spell. Basically any boss with a magic damage component will set it off.
As far as I've seen, there's no real limit to how much damage it can mitigate, it's just a fixed percentage. So, for fights where you're taking very large magic hits, it helps much more.
And although it isn't much damage on festergut, I'd say it shaving off some damage during the fight is worth knowing.
A small note on your build, I'd shift the extra points on improved rune tap or improved 2hand weapon specialization into morbidity. Although the extra threat isn't truly needed, the additional utility of having Death and Decay up is a godsend as far as oh crap moments.