RE: Maxamps.com WARNING
You're cutting yourself short by not running it down to 3.0V per cell. 3.0V cutoff is fine provided they are balanced. I don't know of any lithium battery where the discharge curve is flat. LiIon and LiPo are nowhere near a flat discharge curve. I run 3-cell in my off-road buggy and even there the drop in speed over the course of the run is noticeable.
20 minutes brushed runtime is very good, you can't hope to do much better than that. With a brushless system, you can get 35 to 40 minutes on a flat surface.