"The oxygen flow rate for four people, in these unpressurised planes, at 25,000ft, would be eye watering even with the Mountain High electronic demand regulators. You have to use masks, not cannulas, and in the end this does not represent any kind of practical mission capability."
Point taken, IO but you would file at a somewhat lower level (FL100-160) and climb above the Alps/weather etc.. for as long as you needed (say an hour or two) before coming back down again to your preselected cruise.
Nice to have access to that extra height band and be looking down on the clouds, not bumping around in the grey sleety stuff.
The alternatives for getting up higher (TBM/PC-12/Kingair) are an order of magnitude more expensive to buy and run (with the exception of the JetProp DLX which looks a bit of a chop-job in my opinion, costs far more per mile in fuel has far less payload/range capability).
SB