The sounds are not the same. My church has a PC88MX, I have both an SP88X and a PC2 - the PC2 piano is considerably better sounding. According to Kurzweil it is the "triple strike piano." (means that they have three sets of samples at different key velocity). My K2661 synth has the single strike version of the PC2 sound, there is a difference (there is a ROM available for the 2600 series to give the triple strike sound, but I don't have it, I figure I can get the sound with the PC2).
The Kurzweil ME-1 module has the same Piano as the PC2, but the SP88X is the same as the older Kurzweil Micro Piano module.
Supposedly, the new stage piano just released by Kurzweil has a new piano sound, and the PC3 to be released sometime near end of year is a new generation of instrument.
I'm looking forward to the release of the new one. Even my oldest - a K2000VP still sounds pretty good.
Jim