The insurance company will view this as fitting 'performance brakes' with the purpose in mind of driving harder & thus increasing the likelyhood of an accident.
This is the way it works & you can argue until your blue in the face that they are actually safer by slowing the car shorter & cleaner but it won't make a difference, these type of brakes are fitted by persons who want to drive that little bit harder.
The only way a policy would be made void is if an accident can be pin-pointed down to the brakes, a tricky business I'd imagine unless the components actually failed due to being 'none-standard'. Quite unlikely, especially as an insurance company would be happy to take £30-50 to cover this eventuality.
Agw, I don't believe it would. You can have work carried out by Tom, Dick or Harry and as long as the parts are standard or if not declared then the insurance company has no interest.
If Tom, Dick or Harry changed your brakes and then you had an accident which is pin-pointed down to brake component failure, it would be your goodself who would be taking Tom, Dick or Harry to Court for compensation etc.