It does slow you down about 5% and you lose some rudder control. But I'm trying to think of a scenario where this matters -- wanting to go fast with an evolve installed and not in use.
The rudder mount evolve works very well along with the Mirage Drive as a hybrid system. With only about 70 watts of power while leisurely pedaling you actually boost your speed, can have a 20 to 25 mile range (full battery) and excellent rudder control. Your control console informs you of time/distance remaining so it is almost impossible to unknowingly run out of power. If trolling with the motor off, speed is usually not an issue (you would want to stay under prop freewheeling speed of about 4.5 MPH).
If your evolve breaks down someplace, you have plenty of capability of returning with the Mirage Drive at a productive pace, even against wind and chop. If you're planning to go out without the evolve in service, you can switch rudders ahead of time.
As a matter of practicality, I don't think it will be much of an issue.