I’ve helped out two other ebike friends with the BikespeedRS dongle. One of them has a performance line cx motor and has about 4000 miles on the motor with the dongle. He hasn’t reported any anomalies and the bike is working great.
After speaking to the guy that makes BikespeedRS, he’s confirmed that the unit is only a top speed unlocker and does not attempt to fool the motor into outputting more power. I think it’s safe to assume that you’ll get the power and speed of the cx motor while dongled.
When I compare the performance speed motor to my friends cx, the thing to keep in mind is the maximum power output of the various Bosch motors is only really achieved with a high cadence and or moderate to high effort. I felt that the cx motor had slightly faster acceleration but not my much. While I haven’t tried the cx motor on any real mtb trails, I do know that even the performance speed motor that I have is able to climb some steep fire roads with 60nm.