Spot on, Viv. Experience tell us there's a good chance that a nicely related, moderately performed stallion will fail. That may be in part due to lack of opportunity but, as mentioned earlier, Cullen didn't serve top-shelf mares in his first seasons but still was a whacking success.
The reality - proven over and over again - is that full-brothers may be genetically identical ... but they don't produce identical performances (both on the track and in the breeding barn).
Certainly there's a science to breeding - but there's a whole lot more to it than that.
Michael, you were looking for feedback from people with experience in breeding. I think you've achieved your aim.
Good luck