I don't know which controller Yamaha uses but most are programmable if you have the software and/or programmer. They do not have the plug like an EZGO to upgrade speed.
The only upgrade for speed I know of is the D397 GE 4.3 hp 3500 rpm motor, it should give you about 23 mph on the flats. People who have done this have complained about shortened battery life.
The controller that is stock is only 275 amps.
I would start with a 4ga cable upgrade and see what that does.
Adding 20" tires in lieu of the 18" will give you a couple of mph.