when i had that setup i got about 14 min safely with a 1320, 16min was stretching it. i also got 12 min of safe flying with the 910 battery. i usually fly the whole time with my brushless tail without worrying about heat, but when i had the gws dd i needed to land half way through to let the tail cool down. i'd imagine the stock tail would be even that much hotter.

btw, sounds like you have a good plan to go brushless. as far as the esc, i'd get the cc phx10. it's worth the $$$ for the usb programmability, upgradeable firmware, and of course a very useful governor mode. the governor mode will not be useful with the stock tx, but it i guarantee you'll want it if/when you upgrade the tx. personally i like coolrunning heli esc's governor better (they respond lightning fast compared to cc phx), but it doesn't allow all the epa/timing/startup/brake options the phx controllers have.