Joe:
Great pics! What kind of climb rate were you seeing approaching 18k? How do the controls in a supercub feel at that altitude?
The highest I have been in my spam can is 13.5k tapping the bottom of the jet stream for some serious ground speed and it got real lethargic in both climb and the control inputs there. A couple downdrafts encountered in cruise I couldn't climb through, and had to run at Vy to maintain what altitude I could until I could get out of them. 185 to 192 kts level flight sustained groundspeed in a 172 were way cool though.

Never really thought to push it higher.....not sure how well she would have done.
I'd think that the PA-18 airfoil might hang in there better, especially coupled with the lower wing loading.
Flynengr