Ive used a 5000 mah battery and a 4000 mah shorty and havn't noticed that big a difference in handling. I just give it more tweak on the opposite side to comp for the extra weight. some balance it out, but I was trying not to be a lead barge ; ) The TA06 is a nice car, I didn't want to deal with all the drive line drag from the extra gears and jack shafts. If you run mod you can just run a hotter motor to overcome the drivetrain.
The TA05 in the right hands is a formidable weapon, It won at Tamiya Raceway several times recently over higher end cars such as the 417 et al.
I think tire choice is probably the biggest game changer you can make on this car, that and lots of practice time!
good luck with your choice!