TheoA,
When the wind picks up, I have bad (oh - I forget the term...) steering. I have to push the tiller (vice pull) to keep my course because of all the sail up front pushing the boat down.
That is an interesting point. When I went out on a demo sail with my boats previous owner, the spinnaker seemed to pick the fron end of the boat UP, certinly it didn't drive it down. Could this be because of the way it was rigged?