I have some questions about this.
But first he needs to define rich and lean.
A lean mixture, in my book, will light off quicker because the molecules are smaller.
A rich mixture will burn longer because the small molecules will burn first then light off the bigger ones.
We can't have a lot of timing at idle because it will cause kick-back a gainst the starter.
If vacuum advance is good at idle then if you run 12 degrees mechanical and 15 vacuum, set it all at 27 without vacuum and try it.
I say the vacuum needs to be there at light loads to start the fire early and create cylinder pressures that aren't good at full load.