From where I stand, there are no needed upgrades to a Tamiya. I understand the desire for bearings over the bushings, even though the actual wheel speed isn't that great to cause an issue (again, mine is over 10 years old, still rolling strong with the bronze bushings).
Anything else is completely superfluous, the Tiger is a well engineered kit. If you were building a WSN T-34 or HL, I would definetly recommended bracing and upgrading idlers since the kit parts are junk, but that's not the case in the Tiger.
I you want to spend the cash, it won't hurt, but short of switching out or gluing the clutch gear in the rotation and elevation, you'll have years of trouble free with what you have in the box.