Originally Posted by Corse-R

AFAIK... on the Toluca 1/8 worlds (the highest altitude where a 1/8 worlds has been done) almost all the official guys went with different combustion chambers. I don't have more information about this, but seemed that shape and size of the combustion chamber was severely reduced to overcome the very thin air there and trying to regain more power...

a 15% rel humidity is quite low... I never run an engine with such dry air (my lowest was 24% taken this value with an hygrometer). Maybe what you're experiencing is a very extreme example and the engines need further tunning just to have decent perf.