FX maybe your just reading the word tuning wrong... I wouldnt class a pinion and spur change as motor tuning
but anyway...
generally people arnt talking about 'illegal tuning' opening cans etc isnt typically what this thread is about, its about legally improving the performance of the motor, using techniques within the rules, which DO have gains. To the simplest form, your brand new 540, flush it out with some cleaner and run thinner oil. this is a 'tune up' to allow the motor to run with less drag, = better efficency = more speed, and then you can continue to develop on that by doing a wide variatey of things, using different liquids, and run in methods to gain the edge. this is tuning, we're not talking about modifying the stack, cutting the comm, changing brushes, removing winds. the motor stays sealed, and alot of gains can be made.