From sailing a Tornado the last seven years, I would say not much. I think it would be easier to make the boat go really well upwind with daggerboards replacing the centerboards, especially in sub-trapeze conditions. The difference in L/D would of course make it a bit faster upwind, but not that much. So for a changeover I think we would see a better VMG to windward. How large?? Who knows.. There have been tests done, but it has not caught on.
Downwind I dont think we would see much difference in speed/VMG.
I think sail development have been far more important for the performance of the Tornado today. Just look at the mixed events like Texel and the results the boat scores without olympic crews.

Instead of souping up the Tornado platform further, which is so very dear, I would like to see a new 20 foot platform emerge based on the same parameters as the Tornado/B class. Put a Tornado rig on it, and you might have a really fast platform. Looking at all the "Tornado killer" all-carbon projects out there, I think where they miss out is with the rig. The fast and easy way to get a fast rig is to simply prop a Tornado rig with Tornado sails on top of a new platform.
Sorry if I hijack the thread <img src="http://www.catsailor.com/forums/images/graemlins/grin.gif" alt="" />