Yesterday I finally got around to reading this post (guess I was wondering how you could talk about this subject for more than 200 posts, now I know
Anyway here is my two cents worth. For five years I owned a full length keel Roberts 45 Classic. Below is the design.
Two year ago I sold he Roberts and brought a fin keeled UFO 34, as per below:
I have compared the two yachts extensively in a wide range of wind and sea conditions, including tacking angles, upwind / downwind / reaching performance, effect of waves, etc using the same navigation software / GPS setup.
As would be expected, the UFO 34 certainly had better upwind performance. However the truth is the UFO 34 has better sailing performance in ALL conditions, be it upwind, downwind reaching whatever. I've yet to find conditions where the UFO 34 does not perform better than the Roberts 45.
Some people will probably suggest that this is an unfair comparison comparing a heavy cruising ketch with a medium cruiser / racer. However you also need to remember that the Roberts 45 is more than 10ft longer than the UFO 34 and the larger yacht still loses in a sailing competition. (note 45ft excluded the bowsprit and davits, overall length was closer to 55ft.)
I would also agree with others that point out that the keel type is one of a number of factors that go into sailing ability. However most full length keel boats also tend to be heavy boats with similar charateristics to the Roberts.
Like others have said, there is a reason why Hallberg Rassy, Malo and other builders have progressed from full length to fin keels, as the benefits outweigh the negatives.