By the time I spiraled down through closely spaced thunderstorms in Florida I had spent about 2 hours between 16 and 18 thousand feet.  It was a fairly high stress period.   

 

at least you didn't attempt the 'J-3 spin down thru the clouds' maneuver!  I've done a 6 turn spin in the RV and wouldn't recommend it.

when I first had my -6A flying I decided to fly over mt rainier(14,200'?).  I didn't have oxygen then, heck I had climbed the mountain, certainly I could simply sit there and fly.  then I had another brilliant idea and decided to see how high I could fly, since I was already at 15,000'.  at 18,100 I decided I had broken enough rules and pointed the nose at mt st Helens to go home.  Vne gave quite a steep attack angle.  it was hazy that day and at some point I realized that I was flying towards mt Adams, not st Helens,  about 75 degrees the wrong direction!  yes, I was still real sharp at any altitude! :-)

maybe there is something to that oxygen rule!  the body changes day to day also.  one time I could definitely feel the altitude at 11,000', which has never bothered me before.  it is kind of funny to think of a Floridian needing oxygen, where the highest point in the state must be a highway overpass!  :-)