The only thing that I see missing is accounting for the voltage drop of the A123s under load. Running them in a 1P configuration is going to drop the voltage to a point where you aren't going to realize the rated capacity of the A123 pack. I'm thinking that your runtime is going to be more like 6 or 7 minutes, but that's just a guesstimate. I'd go for the 2P setup. It's an extra 280 grams, but I think it would be worth it and you'd still reach your goal of a milder race setup.