I think the best way is to adjust the diverter omega that is in the XML that is saved into the script. Omega is the same thing as strength. See the line from the script below:
' <diverter mass="1000000.0" omega="35.0"></diverter>
You can adjust the 35 to change the strength. According to JLou, the invisible rubber posts are unaffected by the settings in the XML. If there are other diverters on the table, the changes will affect them, but to my thinking, diverters don't need a strength adjustment to change the path the ball takes on the table. I am uncertain why diverters were ever given an omega adjustment in the first place (maybe to change how fast they operate?). Don't try changing the slingshot settings in the XML. I have tried that with poor results.
I have tried adjusting the Swing Angle on the diverter but it doesn't seem to help all that much although it does change it a little. I haven't tried changing how close diverters are to the Sling rubber. I assume the further the diverter is away from the rubber, the weaker it becomes.