What cells is the pack made from? This is important, because it will tell us whether you can save any weight by going to NiMH, using a smaller cell.

According to the AstroFlight website, the geared Astro 15 cobalt draws about 24 Amps on 12 cells with a 12x8 prop. I take it that this is about where you are? Max current for the motor is 25 Amps, and you don't want to push Astro motors beyond their specified capabilities for long.

On a plane with full Sub C cells that draws less than 40 Amps, you can replace the pack with a pack of HR4/5FAUP cells. This will give you approximately the same weight as LiPoly, without the issues LiPoly brings to the table. On 16 cells, its a savings of 8 ounces over Sub C cells, so I'd expect you'll save about 6 ounces on a 12-cell pack.

Astro says 325 Watts. With the lighter cells, about 4-1/2 pounds. That's 72 Watts per pound. It'll fly "scale-like," not real fast. Climbs won't be that great either.