[Dead Parrot]

Once the throttle is advanced enough to uncover the port source, pretty much the same.

Another benefit of manifold vac advance is lowered risk of running off the cruise part of the carb while at idle. A symptom of being on the cruise part of carb at idle is the idle mix adjustment has little effect on idle speed.

If you want to try manifold vacuum, tie in to the tree behind the carb that comes out of the intake. Be sure to plug the ported port during the experiment.

Ported vs manifold shouldn't make any difference to your originally posted problem as when you are at or near WOT, vacuum advance is out of the picture due to near zero manifold vacuum. That's where your initial + mechanical fully takes over.