I am interested in how other companies measure successes of their high prospects. How do you measure/ensure ROI? What metrics are used?

Could you be more specific? Do you mean "prospects" in terms of "sales prospects"? Or "prospects" in terms of "high potential employees"? Or "candidates for employment"?

Also, you're asking several things here:
--metrics (what gets measured, especially in terms of success)
--ROI (I can get a prospect but spend so much time and resources doing so that it has a negative ROI--so it's not the same as whether or not I was successful in getting that prospect).