I have. I found a cool place that did lactate threshold (via constant blood sampling) Dexa fat scans and VO2 max).

The thing is you can use RPE. We basically mean sustained 4/5 effort (rating of perceived on a scale of 1-5). It's a good enough proxy. VO2 max is also tricky because some people have higher or lower VO2 max but go faster or slower. Higher max != Faster times.

I think the sweet spot for very low carb diets is ultra marathon where you keep your HR around 130-140 anyway, wherein you would use fat anyway. Simple fact is you can't max your power output in an aerobic capacity without carbs. This is not disputed. What we want to know is where the cut off point is in terms of percentage of VO2 max.

Edit: just adding, Noakes talks about ultra performance in his book. It covers everything I saw in that book. He dives deep into the body systems and energy production of the body to suss out the science involved.

I have tried it too. I did not PR in the 5k I tried it for. I think on long runs I never noticed, but 50 minute sessions with 5k pace intervals never felt the same zip as I did on carbs. I was consistent with it for 3-4 months so I am sure I was fat adapted. I noticed my water weight was less unstable (my weight didn't fluctuate as much day to day.) I still like periods of low carb, but I found it onnerous to stay I'm that 20g or less range always. Especially with kids and a wife not on the same diet. So I am optimistically on board the low carb diet scheme, it was fine for me. I cautiously think fine for almost everyone. But... :)