4s 8000mah batteries are not going to provide much more runtime then the 6s 5000mah.
(assume 3.7V per cell "nominal" voltage then 4x3.7Vx8A-h= 118.4Watt-hr
and 6x3.7Vx5A-h=111Watt-hr, only a 6.7% increase in storage)
They end up weight about the same because the store about the same amount of power...

what size battery area do you have to work with? could you put the pack(s) on their side to increase size? (aka do you have the height available). Any pics of your setup?

If you can fit it Mike has a 6s 6000mah battery for $309, my rough estimate is it should give you 30% more runtime (due to more stored capacity). It is pretty sizable though.... 168 x 48.5 x 66.5 mm

here is the link http://www.rc-monster.com/proddetail...pq25c6s6000mah

BTW if any of the battery Gurus disagree with my math/reasoning, please correct it. Thanks