Kirk - you raise an interesting question that has a lot of answers - since there are a lot of variables.
Let me guess - would you be interested in maximizing the transfer of energy - as in silouette shooting? Or is the issue dealing with where the bullet fragments are going after the impact?
Start with a couple of observations. When the bullet hits dead on (normal or square to the metal) it deforms getting larger and shorter; whereas if hitting at an angle it deforms and skids. If it is going fast enough there will be penetration which will melt a good portion of the lead (as in a .223 going into a 1" thick piece of steel - leaving about a 3/4" deep x 1" diameter hole coated with lead) There is real danger of a jacket being turned inside out and coming straight back as well - don't ask how I know.
Dr Mann (the Bullet's Flight) has many pictures of lead bullets (generally soft) and studied external ballistics. Worth looking into for the details of how the bullet files, some information on what happens to the bullets in the process.
So from here experiment - velocity and hardness make a difference. Pick a hardness and run through a range of velocities, taking good notes and pictures if possible. Then up the hardness, repeat.
