|I think that you would need quite a lot of ballast to have a significant effect because the skeg is not far below the waterline and this may affect the hydrodynamic parameters of the hull. |
RL28s sail fastest with plenty of weight on the rail. Feed the crew cream buns or try to get extra crew especially for heavy weather racing and I think you will go much faster than with the lead.
An alternative to consider is water ballast. This puts the weight more where it is needed so much less is required for the same effect. Australian Yachting July 2000 has an article. If you try water ballast please let me know how it goes.