Reporting on anonymous vs known leads

I want to be able to report on the statistics between anonymous leads being created and leads becoming known in particular time periods. I can get these figures in smart lists (using Created At vs Lead Created) but I can't find a way of creating a report that can use these smart lists as input to compare the results. I know Marketo is weak on allowing calculations on the fly, but was wondering whether anyone had found a way around this?

Erica