Regarding Oxygen, I too have the mountain high EDS system with 11 cubic feet
composite tank and would recommend this to anyone.  The best part about it is
that you feel great at the end of the trip.  I used to think that tired
feeling at the end of a commercial flight was the pressurization difference to
sea level, but now I know that it isn't that at all.  It's the lack of oxygen
your body gets on an airliner that wrings you out (maybe the alcohol too).
Regarding refill costs, I've found a friendly mechanic who will do it for $10
and also a way to set up my own transfiller system in my hangar if I decide to
do so.

Frankly, for you shallow breathers, I'd say the test is simply:  how do you
feel at the end of the trip?  If you feel great, or noticeably better than
without oxygen, then throw away your monitoring devices and call it good.  I
have no idea what my blood ox level is, but I know it's far better with
oxygen.

Best,
Ed de Chazal