Ki damage is straight max damage without the +5 or +10 from PA/IPA.
So, for a weaponmaster weilding a copper longsword with a rod-4 with a 30 strength who has epic weapon specialization (+6) the normal damage would be...
minimum damage on non-critical hit:
1d6 + 10 (str mod) + 6 (EWS) + 1d8 (rod 4) = 18
maximum damage on non-critical hit:
6 + 10 + 6 + 8 = 30
minimum damage on critical hit:
= 18 x 3 = 56
maximum damage on critical hit:
= 30 x 3 = 90
Using Ki-Damage you are guaranteed 30 or 90 points of damage depending on whether you rolled a crit (which as a LS WM > level 7, you have a critical range of 15-20 which isn't too shabby). Instead of having a variance of damage between 18-30 or 56-90.
Using Power Attack (PA)... that brings your damage range to:
1d6 + 10 + 6 + 1d8 + 5 = 23 (min) -- (69 crit)
6 + 10 + 6 + 8 + 5 = 35 (max) -- (105 crit)
Using Improved Power Attack (IPA)
1d6 + 10 + 6 + 1d8 + 10 = 28 (min) -- (84 crit)
6 + 10 + 6 + 8 + 10 = 40 (max) -- (120 crit)
In summary, Ki-Damage does not outweigh the benefits of using IPA, but a significant percentage of the time it does outweigh the benefits of using standard PA. And since you get Ki Damage 1x/day per WM level it can add up over time.