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 actually spent a fair bit of time delivering VO2 max tests to athletes (in a previous life as an exercise physiology lab volunteer).
This comment has me wondering if I still know any active researchers in this area, looking for ideas...
https://www.google.com/amp/s/www.runnersworld.com/newswire/d...
It is still unclear, and dubious to claim, that you can operate at 80+ percent VO2 max on hard work outs. Maybe for short workouts or workouts with bursts of activity e.g. basketball.
You can also undo fat adaptation pretty easily. The answer is it depends, but there is little evidence it has any large negative effects. Questions on peak performance vs "feeling fine" will be interesting to see in studies.