I am thinking about this for my 26D. My thinking is........NO BALLAST!!! Gasp!!!! The deal is, many times, where I sail, Marina Del Rey, there are just a couple of knots of wind all afternoon. If I had a weighted keel (daggerboard), maybe 200lbs, and no ballast, I might be a couple of knots faster, given that the boat would be 1000 pounds lighter. Somewhere Macgregor said each 100lbs of weight you add to the boat costs you 1 knot of speed.
Anyway, If the wind picks up, you simply head into it, and fill up your ballast tank! Then you might even have better heavy wind performance as well.
I would not bring inexperienced sailors out on the boat with no ballast though, and I would not cleat down the lines, just sail it like a catamaran.