I would be cautious of shortening the keel - as suggested, if you need to reduce the draft (for a shallow sailing spot for example) then I would suggest you should get the standard short keel. That way you have the longer keel for when you have the chance to sail deeper water.
With the shorter keel with same ballast weight, the boat will be more tender (i.e. heel more easily) - this not an issue in light conditions but sure is as soon as the wind comes up a bit. Adding more weight to the keel will drag the hull down in the water and make the boat sluggish. If you wanted to experiment with this, try taping a 100 gram fishing sinker on the bulb - the effect is VERY noticeable even if you streamline it...
Most of the performance of the DF comes from its light, deep keel design. A 385mm keel is typical for a much larger boat (eg 900mm to 100mm)