SP1xx is not 15.8. There is no 15.8. I only suggested that as a way to think of SP1xx. They are all 15.7 releases because if we call it 15.8 that has other EOL timeline implications.
SP1xx has new features that won't be in the SPxx releases. If you are on 15.7 ESDx and you want new features, move to 15.7 SP1xx. If you only want bug fixes, use 15.7 SP50 (and later SP60).