Dunno? In theory, you might have to use DNP to delete the Major Boss Entity used to speed those up, but I've never tried it.
Deleting the actual boss entity (0364) does nothing and deleting the actual event (#0301) just reverts the speed to regular ol' Monster X (which is quite boring in comparion to "superspeed" Monster X. Then again, I like "superspeed" Monster X and think it gives my mod something interesting.