To answer as a guy, the right tool for the job.
If I needed to hammer in a nail I could use a pair of pliers but a hammer would make the job easier.
I can do Dressage with my QH but to do the job professionally I'd be better off with a Warmblood type.
Are there exceptions? Certainly but they are exceptions.
Can I make a poster out of this?? Thank you thank you thank you!
Also, you do not need a big name, big money trainer to make it to the higher levels on a nice horse. You need patience, time and a lot of willingness to learn from any source.
When you need the big name, big money trainer is with a horse that can't do the work easily and you need someone else with more experience to push it through the issues it's going to have.
And I am answering the question the thread asks. How do quarter horses do in dressage?? Not well, or not well enough.
If there was a thread about Warmbloods doing cattle penning, I think everyone would agree that generally as a rule in competitive cattle penning, the WB would get it's ass handed to it. Because it's not bred for the sport.
I don't understand why as breeders are breeding horses better and better for dressage and other disciplines people insist in riding horses not suited for the discipline and do poorer and poorer in competition. Swallow your pride and realize that not every horse can do well at every discipline!