I have reported this twice in preview, but hey, third time's a charm!
Basically Panther's Instincts makes no difference to damage done to rooted targets, but if you want full details of how to test...
1. With my Lv80 Warden Ranger
2. Mythic Panther's Instinct's companion equip power.
3. All other companion slots cleared.
4. All equipment/gear that has variable effect removed e.g. Ring of Fallen Power
5. Single target dummy at Caer-Konig
6. Crushing Roots class feature used
7. Strike the dummy with Hindering Strike encounter to root it.
8. Strike with three sets of 3 Storm Strike at-wills (dummy is still shown rooted)
9. Wait until roots disappear and Hindering Strike cooldown ends.
10. Rinse and repeat 100 times
Replace Panther's Instincts with another Mythic power that has no effect on test e.g. Angel's Insight. Repeat the test and compare the combat logs.
There is no difference with the damage done by Storm Strike. It should get a +11% increase from Panther's Instincts.
I don't really care if or when you fix this. I'd really just like someone to acknowledge if there is a bug so I can stop wasting 30 mins of my life mindlessly hitting a dummy when I could be wasting it mindlessly hitting mobs.