Let me add possibility 6) Jean might be a little slower at developing her powers simply because of her split focus - it takes more time to learn French and German than to learn just one of them.
One advantage Rogue has with absorbing powers is she gets memory as well - whereas someone like Synch of Hope needs to learn to use a power (even if they get it at full potential) Rogue gets the knowledge of how to use the power along with the gift, meaning that while other mutants who can duplicate powers might possibly have a higher ceiling, Rogue has a much higher floor than anyone else.