Yeah.... any higher and the stall warner becomes a bit of a nuisance... also the autopilot can be used only in pitch hold, not altitude hold.
In ISA
+10 you are lucky to make FL180 however. Or indeed if there is any kind of downdraught. I tried that
just over a year ago, quite close to Mt Blanc, and I think we got to about FL174.5 before I capitulated. Mind you, there were 3 of us plus the kitchen sink
The "Renault" is actually a super aircraft for European touring. Obviously other stuff goes faster and higher but you pay for it with a lot more juice.