Efficiency means different things.
As regards how much power you get for a specific input of fuel it's probably true what people are saying - above the tropopause you gain nothing.
As regards how much power you get per euro of capital investment - it probably gets more expensive as you go up beyond the tropopause. Less air means a bigger engine to get enough power.
As regards how many passenger kilometers you get per kilo of fuel I still think this leads you to go as high as other things will let you. Temperature may start to rise but pressure (=air resistance) continues to drop all the way out to Pluto. I still think if you could solve speed of sound and depressurization constraints, Boeing would immedately stick 4 GE90s on a 747 and have you TCASing up with the Concordes.