I have two seaducers, a .21 1st style, and a new 67. I run a standard speedmaster 21 rudder on my 21, not cut at all and the boat really likes it. Much more stable down the straights than with a cut blade, since the rudder gave me more drag and kept the nose down. I've tried blades roughly 3/4" and 1" shorter and the boat really jumps around on the straight-a-ways. Now my 67 on the other hand had a standard length 40 size speedmaster blade, and it spun out in the turns with the full length blade. (I'm pretty sure im going to say this right, someone correct me if I'm wrong) With the longer blade actually creates lift in the corners for the transom. (Hope I said that right) Each boat is different though. You may have to experiment. So to answer your question, I believe you will see a significant difference between the 5/8". I would suggest the full length first, and trim slowly.
Garrett Randall