Yeah it would make sense that they have some kind of "soft" cap for Potential (so for an 8 you can definitely reach 8 at full speed, then between 8 and 9 you hit your cap, but you can keep training at a much lower speed until the maximum of the band).
Today my trainee dropped in GS after 69/69/64 minutes in the last few weeks so I can't check if and how much his DMI went up, but that 4k up (out of 1.77 million) last week seemed awfully low. It's possible that it's all right since he's in the C formula and even a full pop in Passing should not change his salary or his DMI (according to coach parrot's formulas).