I fly my Cessna well above 10,000ft frequently. Book ceiling is 18,000 but it will go to FL200 on a good day.
I just use a MH Oxygen kit and canulla.
There is a gain to be had by increase in TAS and lower fuel consumption as well as getting clear of weather that would normally deny a non de-iced single from flight.
However for a non turbo engine the loss of ambient pressure means a lower performance even allowing for the increase in air density and the lowering of back pressure on the exhaust so you need to find the perfect cruise level for each type. My Cessna the perfect balance of altitude and performance is FL120. Getting into the Turbo types like the 400 or SR22 that level moves higher. I am not sure what the full throttle height for them is but would imagine it is around FL120-140.