I think there is a kinda limit buffer that they run into that is all to do with pecentage of the bore that can be filled with valve area and the physical limits of how fast you can move a volume of air through it.

Its not governed by material properties any more, as it was in the old days.

I have dragged that right out from the back of the grey cells so dont take it as gospel. If i have a spare min i'll look it up again.