His power attack with a 2HW should be +3 at level 1, +6 at level 4, +9 at level 8, +12 at level 12, +16 at level 15(Greater Power Attack), +20 at level 16, +24 at level 20
I would also take overhand chop to be replacing the 1.5 STR of a 2HW (comes out to +10). I may be wrong on this but that is...