I use the Xtreme Charger to handle the cells and although I have the balance taps in place I just run them dead occasionally and bring them back to full charge and they balance themselves. I actually picked up the premade A123 packs from horizon (4300 mah) and used as is for the rx (configured as 2s2p) and a 2300 mah (configured 2s1p) for the ignition. However, I've now broked the pack down into two 2300 packs in 2s1p configuration and use only one for the rx and picked up some clearanced Black and Decker VPX packs and made my own 2s1p 1100 mah packs for the ignition. However, on this size of model. I would use 2 - 2s1p (min) but would probably go 2 - 2s2p packs for the plane and just 1 - 2300 mah for the ignition.

There's a lot of threads on the A123 batteries on here. One small issue that some people complain about is that the batteries have a very flat discharge curve (that's a bad thing right?). So what happens is that you can't check the remaining voltage of the batteries. Instead you just have to take the 10 minutes between each 2-3 flights (on smaller planes more) to top off the packs. You charge them around 10 amps for very quck charge!