Optimizing Gel Electrophoresis: Ampere, Volts and Buffer concentrations
As far as I know you can either adjust the ampere or the voltage as
both is dependent of the resistance from the buffer. Which means that
I would suggest you to let the voltage as it is. 
As the ampere = voltage / resistance and voltage = resistance *
ampere and as you can see that the voltage has to get higher if the
ampere is increased and the ampere has to get higher if the voltage
is increased (at least if I understand it correctly) the travel speed
should be increased the higher the ampere is set.
I would neither decrease nor increase the concentration of the buffer. Maybe easily using another kind of buffer would help you. But this depends on the different samples you are running. Maybe that could help you:
Brody, J.R., and Kern, S.E. (2004). History and principles of conductive media for standard DNA electrophoresis. Analytical Biochemistry 333, 1–13.
The resistance is more or less given. Therefore as far as I know: No.