Task Due Date vs. Actual Completion Date

I'd like to be able to compare the Task Due Dates against their actual completion dates. All of my Tasks could be completed within their Estimated Hours, but *when* they're actually completed is just as important. A day behind schedule? A week?

The Tasks currently show if they are Overdue(!) which helps keep things on track. But I'm just as interested in reviewing entire Projects and looking for patterns in missed due dates. Is it a particular Assignee that's consistently behind (or even ahead) of schedule? Or are there trends by Module?

Wanted: a report that shows +/- differences in Task Due Date versus Task Completion Date, slice-able by Assignee and Module.

[entails addition of a new field: Task Completion Date, auto-populated by change of Task Status to 'Closed']