Thanks for the replies guys.

The track is pretty twisty with 1 fairly long straight. Its about 340m in length - the faster truggy drivers are pulling around 48-49sec lap times.

As for top speed, at the moment, if I can just get it to keep up with the nitro truggys on the straight, but have enough juice to run for 12-15 min I'd be happy. I was figuring the same as you Unsullied_Spy, if I gear to keep the speed the same, the 7s1p packs should give me lower amp draw and similar run time to the 4s2p. Top speed is really only useful on the straight. With my nitro ST-RR I hardly ever hit WOT anywhere else on the track.

I'm attempting to use the batteries I have on hand and not purchase a LiPo at this stage as the missus has banned them from the house.

Highflier, A123 cells are 3.3v nominal, and 3.6v charged. Lipos are 3.7v nominal, and 4.2v charged. As you noted tho, A123's sag quite alot when in 1P at amp draws over about 50A. The MMM is rated for 6S Lipo so max voltage of 25.2v (4.2x6). The 7S A123's are also exactly 25.2v (3.6x7). So with the voltage sag, they should be much lighter on the ESC than a 6S Lipo.

I'm currently running 4s2p as the setup is pulling 80A peaks and the voltage sag was huge; unfortunately, the car is pretty heavy with all 8 cells in there. About 200g heavier than my nitro. 7s1p saves on some weight, and so hopefully with the higher v and lower A, the pack should last around the same time :D