Korak - I'll be honest, I couldn't follow all the calculations on your spreadsheet... all those table lookups just killed me.
So, I made my own calculations, ignoring power attack for the moment, simply because it doesn't really matter... it only affects your average damage, and power critical just affects a percentage of the average damage.
I did a quickie, and noticed a slight discrepency between my calculations and what your sheet gave (setting power attack multiple to 0)... something on the order of a 5% difference (just over 1 damage). Not sure what that means.
In the end, power critical seems to add at best 1 damage per hit on average, and that's at high levels where you're doing tons of damage. Even at 8th level doing d12+10 damage, I see a tiny tiny increase in damage, about .3 damage per hit for most ac/attack bonus combinations.
The problem is that power critical will matter at best 2% of the time for a 19-20 weapon... 10% of the time you'll roll a threat, and at best 20% of the time power critical will be the thing that makes the difference. 10% * 20% = 2%.
I say at best, because in cases where you hit automatically or miss automatically regardless of power critical, it does you no good.
So.... yeah, it sucks. Sorry.
-The Souljourner