As indicated by the web site, voltage drop is dependant on the wire, the load, and voltage. You may also note that if you ask it to use greater than 19A on 16 AWG wire it isn't impresed too much as you exceed the thermal characteristcs of whatever wire it's programmed for. Now there are a few other factors to take into account. The ampacity of the wire is dependant on more than just the wire material but also the the insulation. The more insulation the less the ampacity as the insulation will increase the temperature (or decrease the ability of the wire to conduct heat away into the air) which increases the resistance which increases the temperature etc. However on the other side the spikes in current for RC cars are temporary which would mean the temperature would not have the opportunity to build and cause a problem of thermal breakdown. Another issue which I have heard nothing about in the RC industry is the effect of "skinning" on the wires due to the high frequencies used by our speed controllers. This would mean that the electricity would be carried mainly on the outside "skin" of the wires which further limits the ampacity of the wires. This is coroborated somewhat if you have read Big Jim's theory on brush serration, he says that the current likes to flow at the edges of the serations to the commutator which is why serrating the brushes seems to be so effective.

I use 16AWG wires on my 12th car, I have taken to feeling the power wires to see how warm they are after a race, I have had no problems with excessive heat so far. This I think is a very good indicator of potential problems in the wires, either ampacity or voltage drop.