@TommyDee: it's more complex than with the MLFB here. First of all, any Li-Ion cell operating at its CDR will have a shorter service life and will lose performance pretty fast. So we always leave a margin. Keep in mind the CDR will drop as the cell ages and its internal resistance rises, so your computation based on a new cell will need even more headroom to cope for that.
Then in a regulated mod, be it in power or TC mode, there is a DC/DC converter (buck or boost it doesn't matter), even if it's pretty good, it's never 100% efficient (say it's 90% efficient for instance) To keep the output power constant, with the cell voltage going down over time, it will need to draw more and more amps out of it. So the longer you discharge the more you put strain on the cell.
Thirdly, most people tend to use LG HG2 or Samsung 30Q over here, and those are only 18A CDR cells, not true 20A ones, like the 25R or the HE4 for example.
All in one, when you factor in the voltage sag which is pretty big at these load levels, your cell voltage will drop pretty fast to about 3.3V under load (ex: 0.2ohm and 50W max power). It will be outputting close to 17A with these example parameters, so really close to the CDR.
If you want to play with numbers there's a nice drain calculator here >>
http://www.steam-engine.org/batt.html?b=LG_18650HG2&mah=3000&c=6.666666666666667
