Hello,
yes higher capacity would be nice.I'm using 6S2P with great results,but a 6S1P solution with 5000mAh would be nicer to handle.
For the form factor...what about a semi square solution like the old 3500mAh sub C cells?Not for the size,just for the shape.Easy to handle,still a round sealing area,and the center of gravity is a little bit lower.I'm using the A123 cells in a Mono 2 Powerboat,and so I have the need for a low center of gravity.
Also thermal coupling and cooling would be easier by using a semi square shape and with the more volume we get extra capacity(6000mAh?!?!)

Greets Burkhard