Scott says that an estimate will help in deciding whether we implement this before the end of the year.
I asked on Whova where I can see the delta between R4 and R5.
Lloyd McKenzie says: We won't come up with a comprehensive list of R5 changes until the time R5 goes out to ballot. For now, the only source of changes is looking at Jira tracker items (which is pretty fine-grained). Also note that some of the changes may well change again or even be reversed before the official R5 release is published in a couple of years.
Peter Jordan said: There's also a 'Compare to R4' link at the bottom of each page in the Current Build which is useful if you're aware of where the changes might be.