Most of these issues can be solved by issuing a mach number and an IAS to convert too which I quite often do. With a stream of aircraft ask the first what their mach number is, then either increase/decrease it a couple of points and lock the aircraft behind to the same or slower (within reason of course).
It's just more R/T intensive which when you're busy you want to reduce as much as possible.
Like 1985 says we separate based on ground speed so it's always fun and games trying to get everybody doing the same with all the variables involved.
Good to see the discussion though!