Seems we've been on a tangent from the opening post.
Just looking at the body mechanics I would think that it makes a difference between thumbs up and thumbs forward for Weaver or Isosceles.
If you were to assume the Isosceles "stance" with both arms thrust forward but then attempted to do "thumbs up" it would create a lot of unnecessary strain - if it's even possible. You'd probably have to rotate your support hand back towards horizontal to achieve it as your thumb does NOT bend up far enough to do this with your wrist angled down.
The same would apply starting with a Weaver and then attempting to hold thumbs forward. And Weaver - not just meaning left foot a bit in front of the other - but (for right handed) left elbow bent, right elbow straight/nearly straight. It's much easier for the thumb to point upwards if your elbow is bent and forearm is pointing upwards too.
So it may be that these thumb positions are more of a result of the rest of body positioning rather than playing an active role in the grip. The question may be: so we've chosen a good firing grip. Where can we put the thumbs so that they won't interfere with anything? Where will they be more relaxed? Can we keep them handy to work the controls?
There are shades of grey in between the two "stances". Who knows what "stance" we'll be in if we ever really have to use a firearm but in order to learn I believe you have to have a starting point. Something to analyze and see the wisdom / limitations in. Good idea studying the particulars of grip.
As a previous poster mentioned - you can experiment. It may be that your hand size + particular gun makes 1 hold easier than another. Is the grip too wide for _ ? Do you accidentally hit the mag release when you do _?
In general when I'm experimenting if something creates muscular strain - or is dependent on muscular effort then it's good to remember that it could affect accuracy. If your aiming is dependent on muscular strength (rather than alignment) what happens when that muscle gets tired? Some muscles tire sooner than others.
Didn't mean to write novel. But I too have had question about particular issues of grip - especially when it seems like The Basics of good grip (according to whatever competitor puts out a Youtube video) changes depending on the grip you use. Basics/Fundamentals aren't supposed to change.