First, you mill kill the electronics with an 8.4 battery. They can barely handle the 7.2. Second you are really opening a can of worms with these tanks as you can try and upgrade them all you want and they will still fall very short of your expectations. I have been through the most extensive upgrades and mods you can do just short of paying hundreds of dollars for the Tank Zone lower hull.
No matter what TU's and tracks you use you still have the issue of the road wheels not being designed for center guided tracks.
I even went through the trouble of designing and having center guides and sprocket adapters made and marketed. In the end It takes a huge commitment and a lot of work to make these models even close to hobby grade and if you do not have any real modeling skill and scratch building capabilities, the aforementioned advice to wait till the HL M1 gets here would be your best bet. Run the tank as it is for now and save your money for the much better model coming from HL.