Sorry, I am not so often here and I did not see this thread before, however I answered to Matt in private mail, since he asked me also that way ... but now may be little more:
5cells are not enough for proper work of that system. Reason is simple - not enough voltage. calculation is simple:
5cells will deliver only 5 * 3.7V UNLOADED. As was written, the internal resistance will lower that voltage. Since we need highest power boost (read RPM) in worst conditions, we have to calculate available effective voltage in highest load at end of battery (on end of flight). The effective voltage is no load battery voltage which is 18,5V for 5 cell battery minus resistance loses in the system. Not only copper resistance in motor, but also resistance in battery. Since motor internal resistance is approximately 40mohm and we have 5 cells with approximately 6mohm per cell, plus something in leads and ESC, so the complete resistance is 40+5*6=70 plus "something" in leads and ESC - say 75 mohms. So if peak current in highest load is only 40 A what is very conservative estimation, we have voltage drop 40*75/1000=3V what gives effective voltage 18.5-3=15.5V and will give us 15.5*680~=10500rpm.
We need 11000 for power boost so we are with new and healthy setup under that number, now imagine that we have little bit "used" batteries with higher internal resistance (when I throw away batteries with too little capacity, I often see 10mohms), or we need to discharge it to value when internal resistance goes up, it will hardly pull model also with constant RPM timer.
So that is reason why I use 6 cells - to have enough voltage head room (difference between necessary voltage and available voltage) for necessary power boost. Yes there is a way to make it little more efficient, but on cost of "repeatability". Means if I do not want to feel aging battery, or differences between new and old or good and worse - means if I do not want flying affected by quality of battery - means I want every flight the same, I simply need some reserve = that "head room", and therefore I use 6 cells. And when I hear some power drop, I know that battery is already KO and it must go away.
Sure, there is a way to fly with 5 or 4 cells. Simply if you have 5 instead of 6 cells, what is approximately 1/5 of difference, you can use 6" pitch prop instead of 5" what will give you back that 1/5 difference. And you will also lose 1/5 of that power and thus 1/5 of trust when slowed down. Simple math. And that is exactly reason why we fly 5" pitch props instead of 6".