Wouldn't the preview version have less load than the current version, and is likely to be faster simply because of that?
I don't think that would be an issue, as it should be scaled to the traffic it receives. I could be wrong though.
Wouldn't the cluster have a certain minimum size to hold the index data? So it's not going to start at one server and scale up, it's going to start at hundreds.
But of course, I could be completely wrong, not knowing anything about how Google works.