Do you have any idea how difficult it would be to rewind a brushless motor or change the firmware of an ESC?

Show me some hard evidence of cheating (pictures of a rewound motor, circuit boards and oscilloscope readings of hacked ESCs, races where drivers have failed tech and had their results removed) and I'll believe you. Otherwise this is just trackside rumour which has been around from day one of blinky racing and is mostly a bad reaction to getting beaten.

Or you are racing against people who cheat the old fashioned way, and put a 13.5 motor in their car and cover over the stickers. Or run a bit of boost because nobody is watching (or because they believe the rumours that everyone else is doing it).

We've had the same claims made in the UK, and as far as I know the random impounding/scrutineering of racers equipment at nationals has never found a problem.