When I talk to product managers from Roland, Yamaha, etc. I have been told that the use of data compression is very common. It is a way to get more sampling out of a smaller area. It's the same idea as when Roland came out with the VS880 digital recorder. With data compression you can have more recording time then without because when the recorder see's a word like mississippi song, the next time mississippi is used again it might only record it as "is" and that will be enough to tell the computer inside it is the word mississippi. I think this is much like compressed sampling is doing although I'm not possitive. I'll try to get a better answer for you from my sources at the keyboard companies.
George Kaye
Kaye's Music Scene
Reseda, California