Regarding mandatory visual committal heights - given that ten pilots will collectively have eleven opinions about anything technical, there would end up being so many "exceptions" that the basic rule would be swamped.
Compare the private owner of a well maintained twin in hilly country, with an experienced ME instructor operating at a flat airfield.
A suggested Engine Out Allowance in the POH / FM might work. But from this thread, it seems that most people have 200' - 300' in mind already, assuming no extraordinary circumstances.
O8