I haven't done a ton of research on this topic. Most of my information came from pretty general recommendations which was reflected in durations and the necessity for recovery. So I'd limit carb intake on any run less than 90 min and allow it on anything longer than 90 min. The primary purpose being making sure a person was fully prepared for the next training run. The idea being runs over 90 min start to dip "too much" into the glycogen stores to be fully prepared for the next run. So an allowance of carb intake was made. But runs less than 90 min didn't push this threshold much, and so limiting carbs allowed the body to work some on fat adaptation.
I don't generally recommend doing the glycogen depletion training (runs longer than 120 min without carbs) until it's been shown that other improvements in quality of training won't take you where you want to go. It's just slightly more risky, but can be beneficial if done correctly.
I do remember listening to this runnersconnect podcast on a similar topic:
https://runnersconnect.net/running-...y-training-bob-seebohar-ms-rd-cssd-cscs-mets/
I seem to remember one of the key things I got out of it was that a metabolic efficiency test was useful for in the moment, but that the bodies response could be changed in as few as a few weeks. So with that being said, I was interested in doing one, but cautious about spending too much (since the data could be irrelevant as quickly as 2 weeks later). Now if they had a staggered system where you got to test multiple times over the course of several weeks, then that's something that would be far more interesting. One thing this does reinforce is the idea of carb loading and the idea of fat adaptation 5 days prior leading to potential benefits.
My guess is this will provide some benefits, but likely in most cases only a marginal increase in performance. But, what works for one person may or may not work for everyone. So if you've exhausted other options in improving performance, then this is the next logical step.
