OK, here are some ballpark figures derived from old test notes, taken in the days when I did some pre delivery testing and annual C of A tests etc on these wonderful machines.
They fall a bit short of Flight Manual figures, but then the airframes were not new and I was only self-taught as a test pilot.
Hence probably realistic for Joe Average flying an old dog of an airframe.
SERIES 100 with 514 engines at 18,000kg would maintain 9000 ft with pressurization OFF (spill valves open) at ISA, no ice, one engine feathered.
At ISA+25, same weight, same engines, it would only maintain 4500ft with pressurization off.
The later series such as 200 with 528 engines were a little better but then we got up to the 500 series with 536 engines which could operate up to about 21,000kg but at that weight they were just as bad as the 100 series, especially at ISA + 25. The only difference was that you didn't have to dump the pressurization if you lost an engine, but at those altitudes it was a bit academic.
I flew all variants in high temps and also in icing conditions in the Iranian winters over terrain around the 17,000ft mark. Any ice, especially on the belly, where it sticks like poo to a blanket, and you quickly got back near stick shaker even at full throttle on BOTH engines! We had to plan our drift downs very carefully.
One would hope that Transport Canada requires such drift down planning when flying over the rocks. In doing so, remember that you have to clear the highest terrain in the drift down path by 2000ft, not 1000ft as is often supposed. The only reason I can think of for this is to allow for unknown headwind, but will stand corrected if anyone has another answer.