What is the best setup 8xl and warrior 9920

[RC-Monster Mike]

What slipper setup? Strobe? stock diff(which will fail on 20 cells in time, even with the aluminum cups and cases-though they will hold up for a while if you are just running on the street)?
20/51 would get you around 40mph. Runtime wouldn't be spectacular, though.
On 20 cells with the Strobe and 16/51 gearing, speed would be in the mid 40s with stupid torque and a pretty decent runtime as well. 18/51 would get you to 50+ mph if the cells can deliver the voltage.
Given your lack of brushless "drive time". I think you will be quite overwhelmed with the power of this setup on 20 cells. Your wheelie bar won't stop the truck from flipping onto its back with this power, and the torque will be pretty extreme. I think it is safe to say you will be blown away with the performance(and so will anyone that is watching). You will want the suspension, steering and overall setup to be on the money or driving will be a real problem!