On the DH8-300, it is as Oktas8 has written. On the -400, there is no MEL item allowing a release without working Autofeather function (the autofeather is an integrated function of the PEC).
A windmilling propeller on the -300 about kills all the performance - in straight and level flight, the simulator could be convinced to climb at 100fpm maximum. On the -400 it is not nearly as bad; while the yaw moment is rather nasty, a noticeable rate of climb is achievable. Again, this is experience from the simulator, I have not tried this in real life.