I know that the remotes for the S50 and 100's only had a few buttons that worked on both. I'd guess less compatibility between sportster and starmates.
I am just guessing based on the SL remotes. Someone else may know for sure. If no one else knows TSS would. They may have some spare SP5 remotes around the shop too.
I had an ST2, and it didn't work with anything else that came later. It would work with the original sportster and sportster replay but if anything it might turn the sportster 5 on and off but that's probably about it.
Is there anyway to program a generic remote to work with Sirius - longer range would be nice - even just to have basic control over volume and channels?