Here's a link to a site that lists specifications, you can choose the sled and review the differences. Usually, the SP would have better shocks, maybe lighter exhaust, maybe other accessories, maybe add reverse. You can also look at the Polaris website to see if the part number for the cylinder is the same. It probably is. The XCSP also was the first to get the VES valves, which may be what your friend is referring to. The cylinder without the VES is certainly different than the one with.
X pkg is a snowcheck package Polaris use to offer when pre-season buying.
Special graphics, color matched bumpers, different snow flap & seat markings w/ an X.
The colors were unique from the standard offerings as well. Red/yellow, sonic blue/yellow 2002 & 03 & even black/yellow in 2003.
You could get an x pkg 550 fanner in 2003 for I had one pic is in my avatar.
It had nothing to do with the cylinders. Also pre-season order let you add different shock pkgs, seats, M10, EStart, & REV. A marketing scheme similiar to the graphics packages they offer now on the Shifts & Rushes.
Maybe your friend was thinking XC (non-VES) or XCSP for VES (Variable Exhaust System). The XCSP has exhaust valves.