Thanks for the kind comments!
albertobdq, the car weighs ~ 1200g and minimum weight is 1474g I guess - so the chassis has to be something of a carbon-fibre brick to bring the wheight up
On the other hand, more weight will set the CoG way off: look at the X-Factory X-6: with a heavier battery, it has to be split in a 4+2 pack to move more weight back to the rear end. You'd never get away with the weight of a 6 cell NiXX pack in the piranha's layout because it will move the CoG too much to the front.