Did you make sure that the new belt was tensioned properly by placing shims under the motor if necessary?
How much movement do you have in the new belt if you squeeze it together between the two clutches?
Did you ever check the RPM of your motor?
Have you had someone rev the motor up in neutral while you watch both clutches to make sure they are engaging properly?
When you are testing the speed, are you pushing forward on the sticks hard to make sure that the T20 is fully engaging?
What are you using to check your top speed?
Have you checked the band adjustment on the T20?