The Finisterre Arrival is just over 5 degrees FPA.
In a Dash 8 100 it is done comfortably at flight idle, 900 RPM, with 1,600 ft/min RoD between 160-170 KIAS.
Constant descent profile from LSALT 15,000' down to MDA of 850'.
IAF 17 DME, FAF 10 DME.
Do you know what the performance of the aircraft would be with one or both propellors overspeeding, or in a combination of overspeed and feather?
At 900RPM and flight idle the PW120 is still providing forward thrust.
I suspect the crew would have been more concerned with controlling the aircraft...