for what it's worth - I just went from basic to competent (PR Manager). Did not lose any games in between.
Merchandise went up about $400, and attendance revenue came down by about $150 (despite a higher seeded team visiting the second week).
Does not seem to have much of an impact so I will probably drop back down to basic the next time.
But note that I am but one observation and there are lot of other factors - my prices may not be optimal etc, I am placing players on the transfer list etc. Perhaps there may also be a delay factor involved?