The end result (one file per section) seems good to me.
I suspect that reviewer burden may be the biggest barrier to going forward. Perhaps breaking up the changes so that each new sect1 file gets its own commit, allowing the reviewer to more easily (if not programmatically) verify that the text that moved out of func.sgml moved into func-sect-foo.sgml.
Granted, the committer will likely squash all of those commits down into one big one, but by the the hard work of reviewing is done by then.