From what I have read, there should only be a difference at idle anyway. If you go WOT, then there is little vacuum with either method. If you are at part throttle, then both manifold and ported should be pulling the same, right?
I wish I had been more scientific-minded when I was playing with my cooling system last summer. My car got up to 220 at some stoplights, so I changed my coolant mix and also changed over to manifold vacuum at the same time (I should have done one at a time to see the effects). I am back to ported vacuum now since I tuned the car last weekend with a different plug gap. I put it back on ported since it seemed to idle better (there is a lot of initial advance, which I might change in the future).
I will probably play around with it this summer, since I am considering getting a new radiator but not sure I actually need it yet.