The bigger the pre-sale GS the bigger the GS hit after the sale. But, you can also experience GS drop even when buying a player in GS level 5. Out of 113 players that I've monitored (which is very low amount of data), the GS drop were as follows:
Pre-sale GS GS After Sale How many Players
5 4 2
5 5 8
6 5 6
6 6 2
7 5 9
7 6 17
8 5 6
8 6 27
8 7 3
9 6 17
9 7 8
Even when the after sale GS remains at the level of the pre-sale GS (which is seen only at GS level <7), you are experiencing a drop in the GS (the sub-levels). That is easily shown by monitoring the value of the pre-sale DMI and after-sale DMI. For example, one player preserved his GS after the sale at level 6, but the value of DMI had dropped from 30,900 to 25,700 (which is ↓16.83%). This drop will also affect his performance. I'm trying to say that any GS drop, whether it is level or sub-level drop, is important.
As I've mentioned earlier in the thread, it is obvious that there is a random factor in the formula.