Harry do you believe the erosion of the GB rate to be due to scoring bias (I hope not) or due to the (presumably) tougher competition at higher levels?

Maybe not tougher competition, but a different profile of batters faced.

At each rise in level, the babip on fly balls fall while it holds steady on ground balls. I presume this is because at each higher level, the average fly ball has a higher vertical angle - more homeruns, but easier to catch when it stays in the park. This could be the result of a promotion bias which favors homerun hitters.

But how soon would they have hit f/x in the minors?

As soon as I get a break from coding the forecasts, I would run these tests - compare GB% of batters who were promoted to those who weren’t. If I am correct, promoted batters would have lower GB%, but should remain relatively steady as they are promoted. Pitchers, on the other hand, I was expect to have decreasing GB% as hey are promoted.

In short, similar trends. And you will see more. I’m working on that for Tuesday morning on the main site, including Brian’s question.