It's closer to 15% if we control for dynamic loft, because the "pre-set drill" doesn't really let me hit anything longer than about a 6I and get the ball up in the air enough to control landing angle well enough, etc.
As I wrote, a good chunk of the distance that would be lost (beyond the 10%) is due to the "more than optimal" amount of lag or shaft lean.
There's a difference between one player giving up yardage to another player and giving up yardage to himself. I can say without a shadow of a doubt that if Jim Furyk gave up 10%, he'd plummet down the OWGR pretty quickly.
You're continuing to rationalize here, but you're really not saying anything differently now than what has already been said: the JV swing can get someone to a level of "competency" a bit quicker, but it has a lower ceiling.
I tend to lose focus riding in a cart. I do more chatting and less thinking about the last/next shot.
I also think I stay looser while walking, but that benefit may be cancelled out by feeling more fatigued towards the end of a round.
I'm not sure if any of that translates to a real difference in score.
Either way, I think is more enjoyable to walk. I wish more courses were walking-only.