I miscalculated how useful Precision is.

It's not like I haven't done these calculations before, but I messed up in a previous post. I said that using Precision when you miss by 4 or less means you'll hit 70% of the time with your d10 maneuver die. This is only true if you miss by exactly four. In fact, you'll hit 85% of the time, on average. If you miss by 1, for example, you'll always hit when you use your maneuver die. Since you're using Precision 20% of the time your overall chance to hit increases by 17%.

This changes the chance to hit for a GWM fighter to:

65% (base) - 25% (GWM) + 17% (Precision) = 57%.

Also, if you care to come up with how often you think a GWM fighter gets his bonus action attack from dropping a foe to 0, consider the following. A level 11 Battlemaster will *not* crit at least once 85.7% of the time. The 85.7% of the time he doesn't crit each of his attacks do 12.13 damage.

Max HP and current HP of a foe varies wildly, but if I had a feature that gave me a bonus attack if I dropped a foe to zero I'd always attack the foe with the lowest HP (adjusted by hit probability). E.g., if I were a 3rd level variant human with GWM facing goblins and an ogre I'd always, always, always attack the goblins and I wouldn't even use the -5/+10 part. One regular hit will drop them. Then I'd take my free attack on the ogre.

That being said, let's assume our 11th level Battlemaster is, with his buddies, facing some CR 5 creatures. They average about 100 HP. So a very rough estimate is our Battlemaster drops a foe 12.13% of the time (because he's doing 12.13 damage per attack on our 100 HP foe). In a round where he doesn't crit he has a 32.16% chance of dropping at least one foe. This 32.16% chance occurs on the 85.7% of rounds without a crit or 27.56% of the time. Add this to the 14.3% chance of getting at least one critical and our Battlemaster gets a bonus attack 41.86% of the time.

This yields average damage for our GWM friend of 46.88. This assumes you also use Precision attack on your bonus action attack.