Exit area for cooling should be a minimum of 1.5 times the size of the intake area. You're right on the bubble. Unless things are really out of whack, you won't hurt the motor in one run. Take it up once and see how hot things get. Sailplanes generally spend most of their time motor-off, so I wouldn't worry.

The only thing stopping you from using LiPolys is your wallet. Nowadays, we have high-current LiPolys so not only can you double the duration, you can cut the battery weight in half. For the same weight as NiCds, you can have four times the duration, perhaps five. You can run the bigger prop for a faster climb rate, and still make multiple climbs.

IMHO, the best deal in high-current LiPolys is the Kokam 1500 from Bishop Power Products. Each pack is capable of 12 Amps continuous, so three in parallel will easily run the 15x12 prop or the 16x12 prop. 4500mAh means more than double the run time. Reduced weight means a quicker climb rate. Add a fourth pack in parallel, and you can run that 17x13 prop for nearly three times as long as you could on NiCds.

But... They're expensive, and there are new, more capable cells coming out all the time. You might want to wait. To get the same voltage as 24 NiCds, you need at least 7 cells in series, maybe 8. Then you need three or four of these packs in parallel. At $17 per cell, 21-32 cells (depending on what you decide) is a pretty hefty chunk o' change