Danielle - Once you delete the leads from Marketo, you will lose the analytics, metrics and the activity log on them. Have you considered setting the "Marketing Suspended" flag on these leads to true, so you do not market to them, or even black listing them, if needed. You could further set some kind of a trigger campaign wherein if they did request content, like white papers, case studies etc, you can reset the "Marketing Suspended" flag to be False, so they continue receive subsequent Marketing content.
it seems that for some reports, the metrics remain the same see:
"The metrics in the Email Performance report includes the activity of all leads including those that have been deleted, which may cause a mismatch with the number of leads a Smart List returns. Adding a filter "Email is not empty" to the Smart List tab in the report will filter out deleted leads."