Regrding Horridon's Last Wish, I just did an LFR with it equipped to check out the proc rate. Results are
here. And here.
Got 4 procs on an 8 minute Lei Shi fight leaving it at a measly 345 mp5. Sha fight has it around 835 mp5. Had an unlogged council attempt last night where I had 66000 mana from cloudburst over the first 3 minutes, putting it ahead of innervate at 1850 mp5.
It doesn't seem to proc with any increased reliability from wild growth (I have a limited understanding on RPPM so not sure what is meant to affect it). It seems unable to proc from overhealing which may give it a heavy bias to high raid damage fights (although my results don't reflect this). I did manage to get it to 2 stacks at a few points during the Sha fight.
The end result is that it seems to be a very volatile trinket that swings between OP and useless at a whim making it very difficult to gear around with regards to spirit.