O2 is about 20% of the total pressure of the mixture (15psi at SL) so this is worth 3psi, and to get 3psi with a 100%-O2 “mixture” you would need to be at something like 40k feet equivalent. Here you go:
20% is found at 12000m which is 38000ft.
On the other hand, with nasal cannulae (and even a mask) you will be far from breathing 100% O2.
According to the military doctors from the altitude chamber, a mask is supposed to make you breathe 100% oxygen. Any indications it does not? except of course if the pilot has a (non-military) beard, making a leak.
Cannulae are obviously “leaking” from the start, and so their use is restricted to max 18.000 ft.
The Mountain High masks used in GA do take in mostly outside air, plus pulsed O2 from the demand regulator. I have one of them. You would need a massive amount of O2 to breathe pure O2.
AFAIK the only restriction on cannulas is what is in the AFMS for the oxygen system, and it is only fitted (not portable) oxygen systems which will have an AFMS. The AFMS usually says mask above 18000ft. Cannulas work fine well above that; I have been at FL210 and IIRC was getting around 95% blood O2.
Peter wrote:
masks used in GA do take in mostly outside air, plus pulsed O2
In case of an emergency you also need pressure breathing when you have smoke in the aircraft. The more sophisticated Quick Donning Masks have a switch marked “Emergency” that after actuation delivers 100% O2 under pressure to the mask, used to clear the mask from potential smoke before you start breathing it.
In this league where a range of AVGAS turbos can go to 25.000 feet is it a problem / downside that the DA42 can only go to 18.000 ?
To keep this on topic, what is the highest the SR22T has ever been?
Alternatively, what is the rate of climb at the certification-limited ceiling?
One would hope 25000ft in accordance with the maximum certified altitude (EASA TCDS EASA.IM.A.007 or FAA TCDS A00009CH). 
I suspect it will still go up rather briskly. It’s an awfully meaty engine for a relatively small aircraft.
I don’t really see how interesting how high a SR22T has ever flown as long as it cannot be done legally. As to the performance numbers at FL250 they are really easy to obtain from the AFM which is available online together with all other SR22 manuals.
From memory book performance for the SR22T at FL250 is 220 KTAS (Columbia 400 is 235 KTAS @85% BHP). Since it has the same engine and roughly same characteristics as the Columbia 400 I would venture that best climb rate at that altitude is roughly the same as that of the Columbia 400 that I mentioned earlier in this thread, ie 900 fpm.
I am pretty sure both planes are capable of FL300+
