the ubec allows extra voltage to be sucked out of the batteries and to the receiver to power the servos. It is basically a battery pack except for it never needs to be charged, gets voltage from regular batteries, is smaller, and needs to be soldered to the ESC. Also to use a battery pack or ubec the red wire on the servo wire needs to be cut off. Here are some wiring diagrams on
this website. for gearing on the 8XL i would run no bigger than a 18t but personally i would run a 16t to keep temps down. it could definatly be run on 18t and would probably work good but temps would definatly be higher. As for runtime depending on the type of battery(mah level) some guys here get an average of 15-20 minutes on a decent set of batteries on a stock maxx with these options (decent battery is like ib 3800 or 3300s). Hope i helped.