Are there any studies that demonstrate the amount of correlation between money-raised and exit valuations?
Yes, many. That's what any study of the returns of venture funds is measuring. And since the returns of the top venture funds are consistently at least positive, we can be fairly confident that $7 billion is a lower bound on exit valuations.
Are there any studies that demonstrate the amount of correlation between money-raised and exit valuations?
With that information, it would be easier to judge how informative it is to use money-raised as a surrogate for the true objective criterion.