Does the wattage make any difference to mid drive kits, in relation to speed or does that too depend on RPM, make and manufacturer???
It's complicated. You have to understand the basics first. The maximum output power is what determines the maximum speed that you can go in any fixed conditions. It's determined by the battery and the controller. You can say Volts x Amps x 0.75. On an MTB, you need 250W to go 15 mph and 1000w to go 30 mph. That power is the sum of motor and pedal power, but as the speed goes up, pedal power becomes less significant.
Now for hub-motors. They have a maximum speed, which depends on the voltage, so we'll simplify it by only looking at 36v ones. At that voltage, each motor has its own max speed by design. No amount of power will make it go any faster.
It's because a motor is also a generator. It generates voltage in proportion to its speed. When it gets to its max speed, it's generating 36v in the opposite direction to the battery, so the net volts is zero and power is therefore zero.
At zero speed, it doesn't generate any voltage, so the whole 36v is trying to push current through the motor, which would cause so much current that it would burn, but the controller only allows a fixed maximum current to protect it. That means that the maximum current available is flat at say 15A until the speed is high enough to reduce the voltage enough to cut the current below 15A. After that point, the increasing speed makes the current ramp down to zero at maximum rpm.
What all that means is that you cannot get max power at max speed, and the faster you go, the less power you can get to help you. A lower powered motor with a higher design speed (doesn't generate so much) will therefore produce more power at high speed than a high powered one.
All the same rules apply to crank motors in any one gear, but you can change the speed by changing gear, but each time you go up a gear, th motor's torque decreases. The crank motor gives you access to high power more of the time, so in a race, a hub-motor used as a crank-drive will always beat it when used as a hub-motor. A bike's top speed with a crank motor is only limited by the max power according to the rules in para. 1.
The basic idea is to have a controller and battery that can give the power for the speed you want, and for a hub-motor, you want one that has its maximum efficiency at your modal speed. Max efficiency is approximately at 75% of maximum speed, so if you wanted to travel at 15 mph, you'd choose a motor with a maximum rpm equivalent to 20 mph and you'd have a real life max speed of 17 or 18 mph.
All this is further complicated by the changing battery voltage, which is 42v when charged and 31v when empty. Everything changes in proportion to the voltage.
, in #1 I listed the watts for each PAS and with 48v it's even better. PAS 4 about 600w & 5 900w if you need it for them's hills.
