Making this model run fast with RC was not my target, but able to drive it in any gear, just like you said. I have now added XL-motor, and it drives very well with gears 1-5 (and reverse). I didn't gear it down as far I have understood what gearing means, I have one 16 gear in motor and second 16 gear in axle what goes trought gearbox (center). That said I don't know these things too well, maybe I geared it down accidentally.
I'm running the motor with 7,2V, so it seems to be quite slow. The car is definitely not going fast even with 5th gear, but it is good speed for model that size. And like I said, my goal was not to make it actually fast. I'd say its top speed is quite perfect to drive inside.
What pleases me most is the motorized gearbox, changing gear from 4th to 5th and back works very well while driving. It doesn't change speed much, but it works!
I will try to take good pictures and better video later.