You can always run a bit higher voltage, but make sure your motor controller will handle it. That will help with the motors. I run mine occasionally on 3 cell but I also have 28mm motor heatsinks with fans running as well. This will help the top speed but your controller must handle it. The stock Taigen and HL systems cannot handle 3 cell so you are out of luck there, but you may get lucky with a 7 cell NIMH. As long as the weight is down and the current draw is low it shouldn't overheat it too quick, just add some fans and heatsinks if you can to the RX18. The 380 Banebots motors can handle 12V no problem so maybe try some regular ESC's with a few types of battery to figure out what works for you.
You might also want to think about some better motor controllers and possibly try one of our 3:1 gearboxes. With something good like a Sabertooth my 20lbs+ Tiger 1 can creep just as good as the 4:1 with plenty of speed, however this will cost you a pretty penny to run these. Remember also that more voltage & speed will equal more noise, so prepare for that too. Hopefully this helps!