I do have x2 of Gurthalak LFR Version, and "tested" it a little on Ultraxion.
Main problem that i had was raid positioning (explanation follows), for some reason spawned tenticles were unable to attack Ultraxion due to range maybe and/or inner bug.
After analyzis of recount and WoL logs, i found that theres 0 (zero) damage, or even mention of Tenticle / Other source of damage, so for around 14 attempts we did on Ultra, tenticles didn't attacked him BUT i have seen tenticles spawn but not attacking (either no buffs listed that could mention tenticle spawn).
Proof log, while wielding two of Gurthalaks
Code:
worldoflogs.com/reports/f4tj7xxl0osud4tl/details/0/
Later on, i've joined LFR for Ultraxion, and found that tenticles indeed spawn (dunno rate, its a mess you know) and indeed attacks Ultraxion, but for some reason damage it did was around 1% of my dps, what leads into conclusion that the attack range of Tenticle is short and/or bugged on Ultraxion (i.e. it does attack sometimes, sometimes dont).
Well, didnt had a chance to try it on other encounters yet, curious to see what % of my damage it will do (expecting 7 to 10% according to wowhead)