I tracked rollover in three worlds, and in almost every instance (>95%) of a player still within his development period, the player showed some improvement from the final development cycle to rollover of next season.
I had the data saved somewhere that I would've been happy to send you, but when I transferred files over to my new computer I think I erased it since it gave me the data I wanted.
Bottom line, there is no such thing as a promotion bump, it just brings the placeholder ahead to that point in time.