Are there good resources for comparing performance of Stock Motorola ROMs vs. CM7.1?
One baseline for measuring phone performance is the Quadrant Score.

Here is the quadrant score for Droid X with FroYo and Gingerbread.  

This site shows various quadrant scores for the Droid X with CM7, as well as unofficial versions of Gingerbread and FroYo.  

There are some debates about the usefulness of the Quadrant Score - hopefully since these are comparisons with the same model of phone, they would be useful.