I say give the final a shot, you don't know what sort of run time you are going to get until you've tried.
I would have thought that you could get 25 minutes between the two packs, I use 4S 4350 and get 11-12 minutes on a rough track, so probably more like 13-14 on a smooth one. 5000 is 115% of 4350, so you should (hopefully) expect 115% of the run time (12 * 1.15 = 13 minutes 48 seconds - perhaps a bit exact...) but 12 + 13 and a half is 25.5 so should be enough. Just don't do the warm up bit at the start, turn the car on and stick it on the line with 15 seconds to go.
__________________
|