A level 1 fighter with power attack, a greatsword and +9 atk vs AC 18 would have a 50% chance to get a regular hit and do 2d12+4 damage with a 10% chance to do double that. On the second attack, taken at a -10 penalty, they would have a 5% chance to do 1d12+4 damage and a 5% chance to do double that. That would average 17 x .5 plus 34 x .1 or 11.9 damage on the first attack and 10.5 x .05 plus 21 x .05, or 1.575 damage on the second attack. Together that is 13.475 damage per round.

A level 1 fighter with exacting strike, a great pick, and +9 atk vs AC 18 would also have a 50% chance to get a regular hit on the first attack but would do 1d10+4 damage and 10% of the time would do double 1d12+4 damage plus an extra 1d12 damage. That first attack would average 9.5 x .5 plus 27.5 x .1 damage, or 7.5 damage. The second attack would be at a -5 penalty, so would get a regular hit 30% of the time, and a critical hit 5% of the time, so 9.5 x .3 plus 27.5 x .05, or 4.225 damage.

On the third attack things get interesting. 65% of the time you will be making an attack at a -5 penalty and 35% of the time you will attack at a -10 penalty. So you can take the same average damage as the second attack, 4.225 and multiply it by .65, and then you multiply the average for damage at -10 by .35. Or at least I think the average will work out properly that way. In any case the third attack will by 9.5 x .05 plus 27.5 x .05 , or 1.85. And 4.225 x .65 plus 1.85 x .35 comes out to 3.39375 damage. So all told the damage with the greatpick averages 15.11875.

If you use all three of your actions to attack (and your enemy does not have some kind of damage resistance) then the greatpick with exacting strike comes out on top. But... how many of your turns are going to be like versus how many where you get only 2 actions to attack or are facing an enemy with damage resistance? I think in play it certainly means exacting strike is playable, but it is not likely to be superior.