Daniel Hutmacher shows how to fake row counts using the UPDATE STATISTICS command:
In this case, the only change was that the query went parallel, which introduces a few more operators. However, a lot can change in a query when you scale the volume, and quite often, the entire layout of the plan can change dramatically.
Note that the query optimizer not only considers the rowcount, but often also the page count (which translates to how many megabytes of data need to be moved), so you may do well to include WITH PAGECOUNT as well.
This isn’t something I’ve ever done, but could be interesting in some scenarios, such as finding out how an application will run as the database grows.