<key>ScrollSpeed</key>
Directly beneath that line will be this line:
<integer>5</integer>
The number will be 1 through 5, depending on the setting you used in the MouseWorks System Preferences panel. If you're using this hack, the number will probably be 5, as that's the fastest Kensington permited you to set. That's the "very fast" setting. Change the number to a higher integer. I used 6, just to see what would happen and it seems to be fast enough for me. You might want 7 for the "Rocket" setting. Save the file. Log out and log back in. Done.
I have not gone back into the MouseWorks System Preference since, as this might rewrite the plist file with the "Very fast" (5) setting (the fastest available in the popup menu) -- so set all of the other prefs for the trackball before you do this hack.

