Look at the test reports for the early Spitfire and Hurricane.
Spitfire Mk I K.9787 Trials Report
The Merlin II & III were 1030hp engines at altitude, (16,250ft plus RAM)
but even with constant speed props they were 880hp engines at sea level using 87 octane fuel. in high speed flight (plus RAM)
However the fixed pitch wooden props killed that. For take-off and climb it was even worse, much worse. They may have had 880 hp at sea level but actual take-off and climb was done at much less than 3000rpm or 2600rpm (max continuous?)
Report listed above has the Spitfire using 2200rpm or less at 3,000ft or below and using 6.4lbs of boost. The engine never gets above 2475rpm, in part because the plane never goes faster than 213.5mph below 20,000ft while climbing and turning the prop faster would just cause it thrash the air producing even less thrust even though more power was going to the prop.
The fixed pitch prop even hurt speed at the lower altitudes because the engine could not run at full rpm without loss of thrust.
The two pitch prop helped but not as much as some might believe.
from
Spitfire Mk I K.9793 Trials Report
"for this setting and normal take-off procedure the climbing speed reaches 170 m.p.h. (A.S.I.), and the r.p.m. 2850 at 2,000 ft. The change over to coarse pitch at this speed and height reduces the engine r.p.m. to 2070 at +6¼ lb. boost. Speed should then be increased to 185 m.p.h. - the best climbing speed in coarse pitch."
Rpm in the climb never exceed 2440rpm. RPM in high speed level flight was also restricted some what.
In fine pitch the airspeed is not to exceed 170mph ASI unless the engine is throttled for fear of overspeeding the engine.
Once the constant speed props were fitted climbs were done at 2600rpm and 6.4lbs of boost and level flight could use the full 3000rpm at all altitudes.
The engine was capable of making much more power at the lower altitudes than the early propellers could turn into thrust.