This one should get you over the 15 minute mark easily, and probably doesnt weigh any more than 8 a 123 cells like you are using now. If you are happy with the performance on 4s a123, you will probably want to go down a tooth or 2 on the pinion to get similar top speeds with the extra couple volts that 4s lipo would have vs 4s a123.
http://www.rc-monster.com/proddetail...=PQXP4s600020c