RE: Great Planes Sequence F3A
Have you look and the website, the important notice, They now recommand 14.8V 2200 man 25C, so I think you could get about 8 to 10 minutes flight if you do a little bit of throttle management.
I don't think that putting a bigger batterie pack would help, you will just add weight and maybe have trouble balancing the plane.
[link=http://manuals.hobbico.com/gpm/gpma1575-tech.pdf]Tech notice from the web site[/link]