yeah I had to post it, because it was a bit more than a 2-300 reduction per week lol L:) . In any case yeah I checked all the 31yo-32yo players I've had also in the past and they all had similar patterns, so I could not disprove Manon's and co. statements and ended up agreeing that Proficient GS does not have sublevels. I had 5-6 players that I could check, so I'm now convinced it's a reliable way to estimate if there has been some improvement in the skills.
Obviously below proficient you are completely correct that GS does have sublevels, but it should have always been suspicious that by inverting the formulas in Coach Parrot Proficient GS is always is between 8.8-9.1 (while I can get a 'good' Strong GS at 8.7-8.8). I routinely check the sublevels for GS and I believe the deviation from 9 is due to my incorrect estimations of sublevels and minimal errors in the model's coefficients. I've never got a 9.3 or 9.4, while I do get high 7s for respectable and high 8s for strong.