Henra,
On the other hand I'm not sure if 25.000 ft is really a good idea. If not ditching I would say fly as high as you can. and get cool thin air in the Cargo compartment. I don't get the rational behind the 25000 ft.
As far as I know this is a physiology thing. Somehow the body cannot handle the low pressure above 25000 ft, even while having an oxygen mask on with 100% oxygen. Above 25000ft, there would be a need for pressure suits and pressure oxygen. Anyone able to confirm this for a fact?