I found a thing on the net that shows how to calculate the power at different altitudes.
Check it out.
http://www.slowgt.com/Boards/SlowGTO...1/00000075.htm
I get 397hp for your car with this graph at sea level and 470hp with my car at 6000'
Perhaps beeing above and below the equator makes a difference as the guys from the UK dont seem to have the same power levels either.