I did check there but it only gave basic specs. I'm talking more like some of the stuff you'd find at World War Ii Aircraft Performance or a manual similar to what the USAAF or USAF issued to units for the P-51H at the same site.
Air Force Standard Aircraft Characteristics, Characteristic Summaries, and USAAF Aircraft Characteristics and Performance Summaries are, with a couple of noted exceptions, created using data from Military Flight Tests. There are cases where, if the aircraft hasn't been flown or tested yet, the charts are produced by calculation of estimated data - which is noted in the SAC Chart. Note that this is the same data used for generating Pilot's Flight Operating Handbooks (Dash-Ones). There are cases where Contractor Data is used, (Generally estimations for aircraft where the contract hasn't been let yet) and that is noted in the reports. The data in the SAC Charts, Pilot's Handbook, and so forth is taken from flight test data, and corrected to Standard Atmospheric Condiions, and standard weight categories.
What you find at WW II Aircraft Performance are the Flight Test Reports, which record the performance of a particular airplane under a particular set of aircraft conditions and atmospheric conditions. This performance is measured over measured ranges, using specially calibrated instruments.
Note that when you look into the powerplant performance measurements (Manifold Pressure / RPM / Altitude), you'll find that in quite a number of cases, the engine isn't hitting full spec. (Low Manifold Pressure, or the Propeller Governor acting up.) In these cases, to get the Handbook values, the power output is determined via the engine's power curves, and what the performance of the airplane is calculated from that data and using specification engine power.
Note as well - the Flight Test Reports that you see are a bucket from a swimming pool, as far as the flight test program's data collection goes. You're only looking at a particular snapshot.
Also note that the numbers for, example, range, endurance and climb are, in the Manuals, SAC Charts and other planning documents, downgraded (Generally 85% tested value) to give a conservative answer. (The worst thing in aviation is a fuel tank full of air.)