Nice job presenting some interesting stats to consider.
It'd be fairly easy to check which GS tournament has the highest average value here, which would show the tournament where the seedings are most likely to translate into results. Instinctively you would think one of the hard court slams, at least in recent times, would be highest in that regard, as the rankings are more likely to reflect capability on that surface than on clay or grass (Wimbledon seeding formula wouldn't do enough to offset the fact that rankings rely much more on hard and even clay).
Looking at the plot posted by
and it looks as though AO tends to be highest which might fit that... but only in recent years. The AO shows a markedly different trend than the other three slams, with much lower seed strength in the 70s before it eventually catches up and looks to follow the overall slam trend from the mid-80s onward. It seems to me this would coincide with the status of AO as a "must-play" tournament increasing, and the greater participation rate of top players compared to in the early 70s.
(Incidentally, if it is the case that the AO's movement from low seed strength to high seed strength coincides with the increased participation of top players, that might tell us something about whether high seed strength is more likely to be associated with stronger fields or weaker ones...