John,

In practice a 20 cell NiCd pack is just under 2kg, a 10 cell A123 is 1kg so you should save over 1kg for 8 cell packs. A123 are far more difficult to solder, then you have the added complexity of balancing wires. The voltage drop under load is about the same as NiCds but watch out for the A123 temperature rise as they can very quickly reach their max working temp.