I upgrade to a 500w motor (with appropriate controller for voltage).
The more amps a battery pack is putting out the faster it will discharge, ie less run time. For the same wattage motor a battery setup will deplete faster at a lower voltage because higher amps are being consumed. Run time is a function of how fast the battery is being discharged. (Ignoring overvolting motor to make it spin faster and decrease life) A motor with more torque can be geared for more speed. Speed is the result of correctly gearing the motor. Higher wattage of motor = more torque output. Torque is a function of the wattage of the motor. Upgrading the motor to a higher Wattage would be the only reason I can think of to modify the cart, as installing a new motor with the same Wattage would not improve the performance of the cart. For example two 22Ah batteries for a 24 Volt pack might not fit well onto the cart, while three 15Ah batteries for a 36 Volt pack might fit good onto the cart, making the 36 Volt pack a better choice for the build in that case. Other factors can effect the decision for which Voltage to use though such as the size and number of batteries and how they will fit on the vehicle. Since you are not planning to overvolt the motor then the Voltage of the system will not effect the power of the motor and any Voltage can be used without a change in power. For example a 24 Volt battery pack made with two 12V 10Ah batteries will have half the run time as a 48 Volt battery pack made with four 12V 10Ah batteries if both battery packs are running a controller and motor with the same Watts rating. The battery capacity is determined by the Voltage of the pack and the Ah rating of the batteries in the pack.
A 24 Volt, 36 Volt, and 48 Volt 250 Watt motor will all have the same amount of torque and horsepower, and a 24 Volt, 36 Volt, and 48 Volt 500 Watt motor will all have the same amount of torque and horsepower. If the motor is not being overvolted then the Voltage of the system does not effect the amount of power that the motor can produce, and only the Watts rating of the motor and controller will effect the motor's potential to produce power. It is best to match the Watt ratings of the motor and controller for the highest reliability. A controller with the same Voltage rating and a higher Watts rating than the motor can be used and the power that the motor will have the potential to produce will then be increased, however, the motor will then be more prone to overheating because more current can be supplied to it than it was designed to continuously handle. If the motor is being ran at its factory rated Voltage then the motor's Watts rating determines the amount of torque and horsepower that it will have the potential to produce when used in conjunction with a controller that has the same Watts rating as the motor. I am also trying to decide if I should upgrade the motor to a higher wattage? But I don't know how these factors interact. I don't plan on over-volting but rather use gearing to do it right. So I am trying to decide if I should upgrade the big three (battery, controller, motor) to 36 or 48v. What if you replace a 250w 24v (running at 24v) system with a 500w 24v (running at 24v) system? Will a higher wattage motor with the same voltage have more torque? Longer battery life? What if you replace a 250w 24v (running at 24v) system with a 250w 48v (running at 48v) system? Will a higher voltage motor with the same wattage have more torque? Longer battery life? But how does this affect a scooter or go kart? I understand the basic of voltage times amperage equals watts. I am trying to understand how changing the voltage (upgrade to 36v or 48v) compares with upgrading the motor wattage (500w, 750w, or 1000w). I am going to upgrade it for more torque and higher top speed. I have a Razor Ground Force that is currently 24v, 250w.