performs just as well as any other turboprop in an engine out
It performs the same as any other TP from that era ie just inside pref A limits. They are certainly not in the same league as the Saab2000 which will quite happily overspeed on one sub FL200.
And from memory Lotee is in the Bay of Biscay about 70miles to the north of Spain and it will be interesting to see if they had a life raft on board for crossing it. Which is another bone of contention with ops and commercial.
MAybe the metro pilots can confirm but I would suspect that single engine speed will be about 150knts and a drift down to to about 6-9k ft so they will have had 30mins plus feet wet to get back to safety. They will be cruising at about 180-210 knts and it doesn't take long for the speed to wash back to drift down speeds. So you will need to let folk know you are decending reasonably soon after securing the engine.
So I think the crew were quite within there rights to declare.