As someone who breaks in horses during spring and summer, and work with a lot of troubled horses in conjunction with vets and equine osteopaths, I can tell you for certain that horses that play up in the mounting yard, at the barriers, or pull hard during their races use up so much nervous energy, it clearly has an affect on their results.
Some are so hyped up and fit that they can overcome it, but in the vast majority of cases it's a definite and distinct disadvantage.

It can even be measured by examples.

There is a horse by Commands, which is no longer racing.
She is very very fast and trialled extremely well privately.
On the track she couldn't even place in a midweek meeting.
She used up all her energy in the mounting yard, would spear to the lead in a race and go backwards from the home turn (get swamped). This horse had all the makings of a good horse, but they just couldn't settle her down.

When I was a young lad I used to look for horses "on their toes", now I look for horses that look half asleep. A horse fighting the strapper, forget it - look elsewhere.
Sunline's last race, the crowd spooked her when the course commentator yelled "three cheers for Sunline" She nearly dislodged Childs. From that moment on, she was never going to win her final race.

__________________
RaceCensus - powerful system testing software.
Now with over 400,000 Metropolitan, Provincial and Country races!
http://www.propun.com.au/horse_raci...ng_systems.html
*RaceCensus now updated to 30/04/2024
Video overview of RaceCensus here:
http://www.youtube.com/watch?v=W821YP_b0Pg