A work around for absolute sabretooth speed control is the sendserial or uart commands and the custom movement panel.... I have an example of using the sabretooth with the custom Movement Panel in the cloud here somewhere....

yes i just took at look at it. seems it is pretty much the same as mine i want to my script to be able to pick the speed on the go and with this Movement Panel it cant change without changing the sendserial commands.. my set up now is the script will do a forward command that works through the custom Movement Panel which inside are the the sendserial commands for the drives ( just like yours ). However if a situation calls for more speed i cant do anything more without re doing the sendserial command variable for speed (port, baud, ###)

You can set variables for speed instead of a constant numbers...

Example.... $Motor_one=127 #full speed $Motor_two=255 #full speed

                 $Motor_one=95 #half speed
                 $Motor_two=223 #half speed
                  sendserial(port, baud, $Motor_one)
                  sendserial(port, baud, $Motor_two)

just change the values of $Motor_one and $Motor_two to slow down or speed up the motors... Variables are global so changing these values in other scripts will change the speed (immediately) in the custom movement panel....

I'm not sure what your application is but would adding the daughter board Kangaroo X2 solve your problem? It gives speed control to the sabertooth for axled robots in Mixed mode.