Sgt Zim,
I'm sorry, but the science doesn't support your assertion that "prime is the only cut that will come out tender, even when rare".
Tenderness in meat that is grilled is directly correlated to the muscle used (locomotion muscle is less tender), animal age (more connective tissue is cross-linked as the animal ages) and genetics. Prime, Choice and Good grades (based on intramuscular fat deposition or "marbling") are all the SAME age range. Repeated testing (mostly at the U of Nebraska) has shown that ON AVERAGE grade makes a difference in perceived juiciness, but not tenderness (as measured by trained taste panels and by the Warner-Bratzler shear test).
In fact, longer cooking using a high-heat method like grilling makes meat tougher. The heat causes protein denaturation and cross-linking. That's why a well-done steak is so chewy.
This should not be confused with high-moisture cooking for long periods (like 8 hours in a CrockPot) which causes 1 of the 3 types of connective tissue (collagen) to break down to gelatin. That will make meat more tender, but substantially changes it's character.
I don't mean to start an argument, but there is a substantial amount of hard science that proves what I am relating and I think that it is important to deal with facts.
To offer something in the way of "credentials" to support that I have truly researched what I've written, I spent 30+ years managing meat company operations including manufacturing, R&D and QA. I have a masters degree in meat science and a bachelors degree in animal science with minors in food chemistry and food technology. I also spent a few years on the scientific affairs committee of the American Meat Institute. I had to know this stuff to do my job.
Thanks for listening,
Browningbbr