5300mAh should be enough. I have run 4400mAh on an outdoor track with a 200' straight and a 5.0 motor, with 1 practice lap. But, I don't pull as much from my batteries as top level drivers. For a 5 minute mod run I average about 35A, where as a friend of mine is closer to 40A in a 5 minute run. I went to 5200mAh packs and didn't have any issues even with a couple of practice laps. Do you notice a drop off, yes. But it's mod, you have way too much power to start with!

For comparison, I average about 30A on an indoor track with an 80' straight.