Should I change the Arduino Mega for a Leonardo or a Due to get better performance?
You didn't quite say what kind of performance gains you had in mind. Looking at your code, it doesn't have any particular needs to justify a fancier Arduino.
With that said, I wanted to highlight one particular aspect of ATmega32u / Leonardo that in my view made it a particularly interesting chip for mixed signal application: its ADC unit features a PGA (1x, 10x, 200x) in differential mode with bipolar readings.
it can be quite handy for some applications.
