Well having run the 808 recently for RRCi, I can vouch for the fact that with the Tekin RX8 and T8 19000kv system in we ran standard 10th twin lipo's LRP 5300 and got well over 20 mins out of the cells and the system.
I would deffo do it. As I have a car here sat doing NOWT!
Obviously those wishing to run them would need new speedo and motor, but the price is coming down and they can be picked up for roughly the same as a normal 10th brushless system, so I can't see why people use price as a problem any more. And considering that if you want to do Rallycross competetively, you will spend more than that on engines over a season.
Anything more than 14.8v and you're looking at silly speeds that can't be handled, so I think 4s 14.8v will be a cut off for power.