MI, I think you might be confusing the F1 aeroplanes at Reno with the Unlimiteds. The big difference being that the F1 class runs unsupercharged 0-200s rather than supercharged WW2 fighter engines.
Some of the F1 planes are achieving positive manifold pressure via very clever intake design, but there is still no way a non-supercharged engine will make as much power at 5000' as it does at sea level.