I course dogs, meaning I take sighthounds, dogs bred to run very fast, and run them in field trials and hunting trials, over natural terrain.
It has been shown repeatedly that if one takes track-bred greyhounds - speed specialists bred to run very, very fast, for a very, very short period of time, over a particular artificial surface, banking nearly constantly to the left - and run them over natural terrain in these other sports - they break. Legs, toes, muscle pulls, etc. They are too fast to do something different other than their specialized niche. Is that niche specialist speed factor valuable for incorporating into breeding programs? Certainly.
But it has been shown that there can indeed be such a thing as, "too fast" for what you are asking an animal to do. Yes, it may need "X" amount of speed to be successful, but that doesn't mean possessing "3X speed" is better.
The animal additionally has to have the other metabolic, conformational, and mental qualities that enable it to use that X speed successfully as an elite athlete. Having only one part of the multi-variable equation for success doesn't cut it in real life.
If I need a horse that can run 11-12 second furlongs for specified amount of distance, why should I presume that one that can run 9-10 second furlongs would be necessarily more suitable or more desired for my intended purpose?
Interesting comment on breeding in DRF
http://www.drf.com/drfNewsArticle.do?NID=83893