SAAB 340 penalty for inop auto-coarsen/feather is almost 3 ton (around 20-30% of MTOW, so very similar to the dash). As with the dash controllability becomes very difficult at low speed requiring substantial increase in Vref if the system is inop. The normal SAAB coarsen system does not actually feather the prop during a failure, a computer cycles the prop at optimum RPM for fast drag reduction.
Most piston twin engine aircraft at mid to high weight will have negative climb rates with a wind-milling failed engine (dependant on atmospheric conditions of course).
What I have in mind is that the drag on an unfeathered piston is probably greater than on an unfeathered turbo-prop. But I stand to be corrected.
That would be very debatable as turbo-props tend to have much larger and more efficient props which in turn cause more drag when being driven by airflow. The reason for the piston lack of performance is more to do with lower power output to start off with. ie, a PA31-350 at max weight struggles to climb on both engines, take one away and add unnecessary drag and the result is predictable. Take same PA31 and strap on 500hp turbo-props (PA31T3) and the aircraft performs much better.
To complicate the situation most turbo-props have a power "push" system that allows operation above 100% power for limited periods during engine failure situations, some over 20% additional power.