The pro mode is for the midi interface that you get with the keyboards.  Its supposed to work with the drums as well.
I'm not sure when that will be available as a stand alone purchase.  Initially they will most likely bundle it.
It allows you to assign and map your cymbals to a rockband color.  With the j5 you have, you are already doing that based on what cymbal you plug into the color input.

Since rockband never interfaced with real equipment, pro mode is just a way to do what the j5 is already doing.
In the end,  the j5 or midi pro mode will accomplish the same thing... that is, pads and cymbals mapped to a color used in the game.
The j5 makes it easier because to don't have to program all the midi parameters to get note assignments and crosstalk sensitivity to work.