Not sure you'll find many people to do just the gearbox map. Viper probably will. Maybe MRC, not sure about them.
The main reason for mapping the box is to prevent it from hunting for gears when you map the engine. You can't effectively map the engine without doing the box.
Depending on who does it and what you ask for, then you could also get such goodies as no kickdown in tiptro (this is a big bonus for me, I hated that the car kicked down in tipto) and no TC unlock on lift-off. I also love that with my MTM TCU, really transformed the driving experience for me.
Some people will tell you that you can put the box in 5th at 30mph. Never saw the point, personally.
Still, they are all relatively subtle mods. Don't expect a chalk and cheese experience.