Lower should be better. High is the big problem because that makes it further away from the subject, a significant problem when he PPF drops off so quickly with fisheyes.
Let say you put it in the middle of the room. Roughly speaking, people will be up to ~25' away, which, on a 3MP stream, would be 13ppf:
For more, see our panoramic tests.
Jason, it will work fine from 8', but the question you need answered is what level of detail are they hoping to view. Is this just an overview cam to watch the comings and goings of subjects, or are you hoping to get clear details of the subjects across that whole room?
We installed a Samsung 7010 in a room a little bigger than yours, but we also had other cameras watching high value items with much high PPF values. We also had a WDR cam on the front door to be able to identify subjects as they entered. The fisheye just allows us to follow them easily as they walk through the store.
In our setup, we found that hoping for much detail at all was very unlikely. Our cam was mounted a little higher than yours. I think it was around 9'-10'.