My Planned Twin Turbo Setup

[XR8tt]

Ah, YES elevation does make a differance. A wastegate only make X or XX boost above the current atmospheric pressure that it sees at that time. If you are at sea level (14.7 atmo) and running say 10psi, then the engine is getting an absolute pressure of 24.7psi. now take that car up several thousand feet above see level to where the atmo pressure is only say 13psi, your wastegate is still only going to make 10psi over atmo cause the spring is calibrated to 10psi. So now your engine is only going to see 23psi absolute and therefor make less power so inorder to get back to the original 24.7psi absolute, you have to raise the boost to 11.7psi to compensate for the 1.7 loose in atmospheric....

That part of it yes but the correction in some chassis dyno's is set for N/A or S/C engines...
I could still argue though ? Even IF atmo was 5 Lb lower the turbo still works up to wastegate spring, hence whatever it's set at.. This was the main reason turbo's were designed in ww2 in piston fighter plane engines.. So they could go higher in elevation without loosing power...TMK the T04 design goes back to then!!
What you say above is more along the lines of supercharger where drive is fixed to engine rpm..
If the boost controler is working OVER spring rate ?? Then yes it would be suspect to atmo pressure...