If the internal bec is shutting down due to there being too much heat, I don't see how a cap can help with this. It may help with some occurrances, but will not help with all. If the internal bec cuts power for any length of time, the cap won't be able to store that much power. The cap will help with voltage drops associated with running a low quality nimh pack, but there isn't any such problem when running with lipos.

I'm just sharing my experiences. I use an external bec. They're cheap. A switching bec can be had for around $10.