An interesting sidelight to this discussion comes from the requirement to
fully test the airplane in all aspects of it's intended use. (Can't quote
the regs but I'll bet someone on the list can).

When you are finished with the restriction period given in your paperwork,
you must attest that (paraphrased) you have explored (tested) all flight
regimes in which you expect to operate and have encountered no safety issues
with your airplane.

If you intend to operate in the flight levels above 18K, it seems to me that
you must test your airplane there to make that statement.

Dan Schaefer