I am not sure how much you actually know about micro controllers. Just want to add that something like a Raspberry Pi would absolutely terrible for something like motor control. Raw digital sensors and PWM requires a lot lower level access than an actual computer can provide.
Arduino mega is substantially weaker than a VEX cortex.
https://store-usa.arduino.cc/products/a000067
A Due is to be honest decently close in specs to a cortex. Due is the first arduino to actually allow all digital ports to be configured to act as interrupts, something a cortex has had for 6 years. And the cortex comes with lots of components that are not included on an arduino.
Have you ran into an issue with the amount of power that a cortex provides?