Greg, isn't the "downforce" a result of all factors? Not just the removal of internal and underbody drag/lift?[/b]
Without detailed information, of course it's impossible to do anything but "educatedly" guess. However, I suggest reduced lift from taping up of the cooling duct is primarily due to reduced underbody airflow...

...carefully angled grill...which now acts like a diveplane.[/b]
I'd be real surprised if it actually created much vertical force. But again, I don't know that for a fact.