20-30% off power specs may be true for some engines but the supercharged systems that LR use have a significant amount of reserve capacity. They will pressurize the intake charge to the same boost level regardless of altitude by varying the flow through the bypass valve.
Also worth noting is that it was only 2009 where the 4.4V8 LR3 had just 300hp and 315 ft lbs (at 4000rpm). The 3.0SC is still significantly better than that especially in the torque curve, and being coupled to the 8-speed transmission. I dont hear LR3 owners lusting for more power too often.
Loss of power at altitude is not about reserve capacity. The density of air is much less and atmospheric pressure is 10.1 to 12.2 psi vs. 14.7 psi at sea level. Engines are air pumps and compression >ratio< is simply the pressure above your atmospheric pressure. Same with Turbo's or Superchargers. They are boosting the cooler and less dense air above ambient in order to produce higher power output from a smaller displacement engine. Same with 8-speed transmission, with tighter operational ranges per gear to better match power demand of the driver to a smaller engine.
A quick search found the following on the MB forum:
Brandon Kleemann USA said at altitude a Kompressor engine losses so far between 15-17% of power, turbo engines 7-8%, and normaly aspirated 20-22%. Those numbers are consistent with other discussions I've had with others.
If Land Rover can increase boost by 20% at altitude I'd like to see a document on that, and associated hp/torque curves. I know they have them at the engineering level. I just don't see LR data in this area.
Also, it is common knowledge for those who live at altitude that larger engines get better fuel mileage while the smaller engines never live up to their hype as the driver has to work the drivetrain harder.
As I've read, the LR4 is suppose to loose 800 lb's in 2015. In addition the single speed xfer case will further reduce total weight. Both needed for smaller power plants.