I have just run few test with Wolfshead Helm on training dummies and figured that someone may find the results interesting:
My gear is far form perfect, unbuffed in cat form:
Without Wolfshead Helm (Guise of Midgard Serpent, socketed with +27agi & +21 agi +3% crit dmg meta):
7003 AP
200 Hit (6.10%)
262 haste rating (0.91 speed)
123 ArP (9.99%)
50.31% Crit
19 Expertise (81 rating + 10 from talents)
With Wolfshead Helm:
6607 AP
216 Hit (6.59%) - I used Arcanum of Ferocity instead of Arcanum of Torment which cannot be applied on Wolfshead Helm.
262 haste rating (0.91 speed)
123 ArP (9.99%)
46.77% Crit
13 Expertise (28 rating + 10 talents)
6K mana
The initial tests were 10 minutes long (4 berserks used), without any buffs and without anyone else interfering. The "rotation" was maintained with the help of FeralByNight addon.
I powershifted as soon as I had less than 50 energy, had mana for it and had a free global CD.
lvl 60 training dummy:
Wolfshead helm: 4.6K dps
Guise of Midgard Serpent: 4.2K dps
It looked promising so I tried it on heroic training dummy (boss lvl) with the same conditions. Unfortunately, the results were the exact opposite:
WH: 3.45K dps
GoMS: 3.6K dps
Then I tried shorter fight as I was OOM after few minutes of powershifting and while mana regeneration from ILotP is nice, I heard "Not enough mana" a lot more than I would have liked

Heroic training dummy with 5min test (2 berserks):
-------------------
WH: 3.67K dps
GoMS: 3.51K dps
-------------------
WH: 3.72K dps
GoMS: 3.6K dps
-------------------
WH: 3.66K dps
GoMS: 3.6K dps
I know that it is a fairly small sample of tests - yet WH's worst attempt is still better than GoMS's best.
Few observations:
* The shorter the fights should be more benefical to WH since you have enough mana for all powershifts during the fight.
* The better stats you have (better gear / raid buffs) the more benefical that +20 powershift energy should be (based on similar 2T4 bonus scaling).
* Raid buffs increase you mana pool & mana regen effectively increasing the number of powershift you can take.
* You can use mana potion to make even more powershifts!
* When out of mana, you cannot Battle Ress / Heal anyone, which could be an issue. You can save a mana pot for these situations.
I'm eager to try it next week in Ulduar on shorter fights (Ignis, XT-002, Kologarn, Auriaya) to see if it really is viable.
Can someone with better gear confirm these results?
Edit: added additional test data & reformulated conclusion.