John-Sounds like a plan. Make sure you do not get the Bikespeed confused with the Badass box which is from a company in Germany that will ship to the USA. Perhaps Alaskan can jump in and let us know what his experience has been up til now with his Badass box indicated below.
Alaskan's Badass Box
Great choice to combine both (the dongle) on the CX. The ultimate setup motor would be the torque of the CX along with the speed of the class 3. No one (from what I have seen) has complained about any concern of undue stress on the motors using a speed dongle.
Here is a crazy thought. Swap out the motors between bikes with the speed dongles. Not sure if the bolt patterns and electronics are the same or not. The other part is having a skilled and qualified/certified Trek tech do the job to your satisfaction as well. Can get interesting!