Paul,
Advice to me has been along the lines of - holes reduce disk surface

area,

reduced surface area is less brake clamping area.
George (down under)

Yes but they are widely used in road racing with great success.

Paul Lamar

Paul,
Sorry I didn't say this before but it was put to me that they are mainly for
cooling in extreme/ extended braking conditions.
I can't be sure myself, these opinions are from a disk manufacturer here
locally, that has the expertise.
Just thought I'd tell you FWIW.
George (down under)

I would like to add, that all emergency vehicles in the United States 
either use cross drilled holes or slots for the resistance to brake 
fade. That has been common for numerous years here.
Doug

 
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