[sblock=noise]
Just tweaking the numbers does not get us anywhere. We're just sliding our setting on a scale - no matter what we choose the feat will always be a trap for newbies on one hand, and abusable for the powergamer on the other. Simultaneously.
Yeah, yeah, it's imbalanced. Acknowledged.
Compare to some rare and highly desirable magic weapon that deals +2d6 damage. Big-ass damage bonuses are in the game! The difference is that the game would never think of giving this out without DM approval (in the implicit form: unless the DM likes the item, it will never be found by the characters).
Feats, though, /are/ optional, which means that if you can take GWM or SS (OK, outside of AL), the DM /has/ implicitly given his approval. And, no that list of feats is declared optional as a whole doesn't change that. A DM can examine all the feats before he opts in, and could ban specific feats if he so desired. So a DM allowing feats has given his tacit approval to the 'broken' ones. [/sblock]
signal
I still prefer -3/+6. Maybe -3/+5.
Yes, if I'm reading this right then -5/+7 is basically even without advantage and very good (but not insane) bonus when you do have it.
I'm kinda like'n -P/+
P with n >1, though maybe only fractionally...
The core of the problem is the feat applying an effect to every attack the character makes....
To actually solve the problem we need to abandon the idea that the feat gives a damage bonus altogether.
Or that it gives that bonus to multiple attacks in a round.
For instance, if instead of an attack penalty, the GWM option used an attack action to make a single all-or-nothing attack... that'd fit the idea of an all-out Power Attack, and eliminate the force-multiplier issue. The attack could still be /big/, though, multiple weapon dice, and the +10 or some similarly large bonus, such that the average damage wasn't behind taking the usual set of extra attacks, but the very high single-attack damage could blow through resistance or be particularly valuable any time a damage threshold mattered... or do something, like knock prone (bludgeoning) if you it a certain damage threshold...
...hm... similarly, the SS option, instead of a -5 could apply only to the first attack you make that turn, and only against, say, an enemy that hadn't moved on his last turn. So, you're aiming, see? Makes sense, no? Or it could work just like the above GWM 'fix,' it's one carefully aimed attack /instead/ of the usual Extra attacks, but doing multiple weapon-dice. Conserves ammo, punches through resistance, etc... maybe does something on a damage threshold (different from GWM, or based on damage type...)