there is still no good definition of soft cap... as long as the stat gives more dps than another stat, you want it... you could calculate when the slope of the curve is halved from the initial slope, but that still doesn't mean much to how you gear.

and looking into mastery/power trade would be meaningful if there was a power augment... it is impossible for power to be more than 15% better than mastery ((.23-.2)/.2) so trading 99 mastery and 39 power for 80 power is a loss of 23% (point for point, not in bonus damage), which is greater than 15%, so not viable. also, 138>80 and more numbers is pretty much always better than optimized numbers (except when alacrity in concerned)

in reagards in when to trade crit for mastery, look at the OP. if you have high base hit and surge, you want more crit (likely as much as you have now with no mastery augments). If you have low surge and low base hit, you want to trade for mastery around 2200 crit... but that assumes all the stuff stated in OP.