Does a cheap ($800) weighted keyboard (my Casio CGP-700) put out the same midi data as a more expensive ($2000) weighted keyboard? In other words if I like the action of the Casio would I be missing something if I used it rather than getting a more expensive keyboard and getting used to a higher end piano's action?
There is likely no improvement in the midi data in a more expensive keyboard. So, yes, if you like the action of your Casio you should not bother to upgrade.
In general, differences between keyboards in midi data will be in the particular velocities (0 - 127) that get assigned depending on how hard you hit a key. If you don't like the midi data your keyboard is sending, the solution is not to buy a more expensive keyboard; the solution would be to choose a different "velocity curve" for the keyboard you have. On some keyboards this is called adjusting the "sensitivity" from e.g., "heavy touch", to "light touch". Keyboards generally have a few different possible settings. However, when you get an external computer into the mix (as you are if you're running Keyscape or some other VST), then there are utilities you can use that give you nearly complete control over the velocity curve, can customize it as much as you want; the velocities will then be adjusted on the computer rather than in the keyboard.