Try maneuvering the airplane in simulator at high altitudes and judge your comfort level yourself. See the airplane reaction.
Never mind the simulator. Before the days of RVSM it was not uncommon to hand fly airliners (even heavies like the 747) for periods of time at high level, e.g for trimming purposes or just for the sake of it. At no time do I remember reports of there being severe control problems or severe control issues.
Its very easy to show your macho in the posts here.
I'm all for not cruising right on Max level for the sake of it but as in all things there's a balance. Some would regard it as being "macho" if you decided to stay low to penetrate monsoon weather, rather than being up at a sensible FMC related cruise level.