I don't have this exact set, but I have the 41999 set with the same mechanics. I switched the gears at both the motor end and the universal joints for a double gear up. Without the body on the chassis flies around pretty good. However, there is a significant reduction in speed when the body is on (obviously due to the extra weight). After a few minutes of running it at full speed the overload protection kicked in and the truck just stopped. Turned it off and back on again and it worked fine.
I'll probably end up switching one of the gear ratios back to they way it was so I get a bit more torque, but it won't be as slow as if I had built it per the instructions. I'm also working on a mod in LDD that will use the XL-Motors, but since they spin slower (220 RPM vs the L-Motor at 380), I will have to gear up for more speed. I'll crunch some numbers and let you know how that plays out. Given that the XL-Motor is wider I'll have to redesign a few areas in order to fit it.