I'm not sure about Mtb derailleurs, but with road derailleurs max capacity listed is usually what the derailleur can take -2. So a 28 cap derailleur should be fine using 30t Max cog. However this also depends on the rear triangle geometry on your frame. Sometimes same setup will work on one frame but not the other.
I'm not sure about Mtb derailleurs, but with road derailleurs max capacity listed is usually what the derailleur can take -2. So a 28 cap derailleur should be fine using 30t Max cog. However this also depends on the rear triangle geometry on your frame. Sometimes same setup will work on one frame but not the other.