You're going to get a whole bunch different recommendations regarding bearings, gearboxes, etc. based on personal preferences.
For myself, I built mine stock over 10 years ago, using the metal bushings and nothing else mechanically that wasn't included in the kit. In all that time, being used by myself and my sons, even when they were real young, it still is going strong and remains one of my most reliable tanks. I only lubed the gears twice in that time and the only adjustment I had to make was with the torsion bars, which was my fault. Until last year, I let it sit on it's running gear on the shelf, not on a block like you should, so the bars fatigued. I was going to replace them, but ended up just bending them back and they sit fine. It also has the original kit ABS tracks.
The thing has fought in houses, fields, backyards, rolled off a wall, bounced down three concrete steps, had 1/16 oak trees fall on it (decapitated the commander three times) and it still runs like a top.
I don't think I'd get better performance with a different gearbox, roller bearings or anything else, 10 years of use and abuse without issue, nothing added but batteries.