I am also using an SP5-8 and have the same problem. I discussed this with both AM&S (their repair arm) and Kurzweil directly and was also told that this was endemic to the instrument itself and that taking it in for warranty repair would be a waste of my time.
They suggested I set my velocity higher so that I didn't need to play a note as hard for the same sound! Jeez.
It's a good instrument with a good feel and good features, but I regret buying it now.
Kurzweil needs to do something about this.