I just did some testing with HomeStudio7xl and Studio Instruments Piano. My input was a keyboard synth.
What I observed was that it was velocity sensitive but you had to get way down on the velocity before you noticed any reduction in sound. And then when you have high velocity the quality of the sound changes.
So SI-Piano is velocity sensitive. I don't see a way to change how it interprets velocity (linear, exponential, whatever).
The easiest way to really figure out what it is doing is to input midi notes (like lots of the same quarter notes) and then edit the velocity of the notes to go thru various ranges of velocity.
--Larry