I did a search and found a few things in regard to battery internal resistance. At what most folks fly on this site, the big ones, it may not make that much difference unless your under rated for the setup. I was playing around with one of the calculators and noticed a sizable difference in thrust/speed/watts when all things were equal except the "C" rating was changed. I fly 1/2a for fun/sport and with a change from 20C to 65C with the same motor constants there was a significant difference or at least it seemed that way to me.
Plane, auw 8 oz
20C 3s 450 mAh battery.
Run time  wot 3.4 min
amps, 7.93
Thrust, 18.6 oz
Speed, 45 mph
178 watts/lb

Same plane, prop and motor but with a 65 C 450 mAh 3s battery
Run time wot, 3.1 min
Amps, 8.78
Thrust, 20.4 oz
Speed 47 mph
194 Watts/lb.

The drive weight in the calculator only changes by .1 oz when going from 20C to 65C.
So, I guess my questions are, what or where would be the point of diminishing return as far as increase in c rating and longevity of the battery vs cost on a normal life cycle. Would a 65 c battery that is never even warm after a flight last any longer than a 20 C that is just warm?
Is the motor being starved at a lower but acceptable C rating?
With given/fixed motor constants, is there a way to calculate at what C rating there will be no more change in power output?
How much of the theory will apply to real world results?
Also, how accurate are the online drive calculators we have today?