I think once the ceiling is reached it's the rate and the angle of climb which are affected. And pressure after a certain altitude.

I got up to FL 410 for the first time yesterday.. Ran just fine, but I waspretty light as far as fuel and only medium loaded.. Some flights won'twant you to go that high as far as the FMC.. Probably a combo of weight,wind, temp, etc.. But yesterday I finally ran one that said I was good to goto 410.. So I climbed on up just to see how it would act. No problems..I had one flight I flew to Honduras, and I tried to go a bit slightly than theFMC specified.. The plane didn't like it much at all, and I went back down towhere I started. :/ I think I was at 380, and tried to go to 400.. Didn't likeit that day. It was really straining at 40k. I bet it was the maybe warmerair temp aloft it didn't like.. When I used to fly the Concord on the sim, itwas picky about air temp.. Didn't like warm air much..

i was flying 3 hours at FL500 (just to test some things.. nothing happend and no problems with flying an assigned track)