So the 945 Specter seems to be... good. Quite good. Here's a log of a H Goroth where it did 5.23% of my damage:
https://www.warcraftlogs.com/reports...pe=damage-done
I basically used it off CD - with the 0:45 CD, every 2nd one lines up with the Vanish-Symbols-DFA-Dance combo. And these things CAN crit and are buffed by all of those damage increases. And my usage was far from perfect, too - this was just a pug, and many of my uses did not double hit because of boss movement.
So not only is that a 25k upgrade over my 905 NBF from Agility alone, it seems to do as much or more damage as NBF, even with less than perfect usage.
I suspect it will work even better with 4 pc, where it can be used with Symbols-Dance every time, as every second Symbols is 0:50 apart. It's a tough call whether that will be better than wearing 2pc+2pc though. That sort of thing will be hard to sim since I don't think the APL is clever enough to use this trinket optimally in both setups.
P.S. Yes it's off GCD