TMC wrote:

what is the approx rate of material removal per hour with a hired grinder?

Depends how many sharp teeth on it.
Here's one I did earlier

http://www.youtube.com/watch?v=ZeXVDsgw2ew

No worry about the 2'9" gap this machine makes its own.

The little Dosko pedestrian controlled one costs about GBP250/day and will
take your stumps down to 250mm below ground in that less than that time.

AJH