This is not correct. In the unmodified condition there was no restriction of the maximum boost when the boost control cut-out was used, thus the boost could be increased to about +17 lbs at full throttle at sea level.
Thus the boost override could not be used in the unmodded condition since it would cause almost instant engine destruction, which is a very bad thing for pilots to do during combat...It was only after 100 octane was used that the boost override became a useful adjunct to increase low altitude performance.