Test prep has a small impact ONLY WHEN ALL OTHER BACKGROUND FACTORS ARE CONTROLLED. Many studies have found that without these controls on things like parental income, school district, etc., test prep has a large impact due to the vastly improved test prep of students who can afford high quality coaching/spend lots of time preparing.
If they spend lots of time preparing, they deserve it right? Even if you compare a rich kid preparing full time versus a poor kid preparing exhausted for an hour at the end of the day, the rich kid rightly deserves it - logically they'll also have more time for their studies at the college and do better at it.
I'm all for equal opportunity, but not of outcomes.