Can't comment on MB performance at high altitudes...left all of them at home when I did a 6-month contract in Colorado Springs. Decided to buy a cheap car for driving to work and around town, then sell it when I move back to Texas.
Bought an 86 Chrysler Conquest turbo for $3K. Decided I liked it, so I got it repainted, redecaled, rims polished, valve cover polished, etc. etc.
Anyway, even with the turbo, it was anemic around town. Never got the boost gauge past 0 psi. When I finished the contract, I drove it back to Texas. The car literally screamed when I got to the lower altitudes...boost gauge shot up to 7 psi!
Funny thing is the engine temp was always normal, whether in Colorado or Texas...
2009 ML350 (84K) - Family vehicle
2001 CLK430 Cabriolet (71K) - Wife's car
2005 BMW 645CI (124K) - My daily driver
2012 Mustang V6 (60K) - Daughter's car