Below my experiment to my rear shimano disk brake
what you need actually only steel saw,
make the groove line for 45 degress across your brake pad
And then place it reverse to disk movement
make sure the groove is not deep enough
After trial some riding trips, found that actually no significant increase, maybe riding trips not quite good enough to test the disk brake performance :p
But one thing clearly, it's help cleaned my rotor, brake pad from dust / mud , i just need thin piece of steel stick inserted into that groove