The 2002 ISMM paper on MPS says that there is a global pause. As usual, it may be fine for small heaps and allocation rates, but eventually it will become a problem.
Since we're just moving our commercial clients onto 64-bit, we don't have experience except up to the 4GB limit (or 3GB in old Windows) which they approach regularly. We're still waiting for it to become a problem. I may have a different opinion in a year.
One of my short term goals is just to get some measurements together to publish. Watch this space, and sorry that they aren't here yet.