I've got no data, but I'll throw out personal observation after 3 & 1/2 years as a T-6 IP:
I can count 11 American students I had that I would classify as "full retard." The type of guys I just had no way of contemplating how they convinced anyone that they were capable of doing anything remotely as complicated as flying an airplane. I'm talking about guys that were so bad I wondered why we even bothered with IFS. 7 of them (64%) were AFA grads. I don't know what the percentage of UPT students who graduated from the Academy is, but it's not anywhere near 64% is it? Overall I never really saw a difference between the quality of student pilot between ROTC/OTS/AFA guys, but I considered it bizarre that a majority of the guys that were way out of their league came from the Academy.
Don't know if this adds anything useful to the discussion, but it's always fun to make fun of UPT students.