IF I understand correctly, the long and short of it is:
Upgrade the core, then the MD's would upgrade from the core's repository and not online repositories?
Thus cutting down on th eamount of data that needs to be downloaded?
Yes. Except for any packages that might be specific to the MD, everything upgrades locally.
However Possy has pointed out that when upstream changes break things (like mythtv, asterisk etc), this procedure breaks everything, and we should work on the xml updater. You are free to use this, but it will not be implemented as the default behavior for "force upgrade". It is pretty convenient though.
I should also be doing kernel version checking prior to upgrade commits, as there are weirdo systems out there which have different distro MDs and such.