The quality and depth of the mares you get bred to mean WAY more than anything else.
I'm talking all things being equal - brilliant speed is tops at seperating.
If you take a cheap speed career claimer with no pedigree and breed him to a bunch of mares who are worth only the meet on their bones - you aren't getting much.
If you take a slowpoke like Jazil and breed him to an excellent band of MANY mares - he will disapoint in a big way I believe - but you'll get some runners.