On page number 13c the performance is based on calculations for the proposed type. There is NO flight test data in a detail specification, it is all calculation.
There is a preliminary specification, a detailed specification, a preliminary design review, a detailed design review, and a prototype is authorized by contract. No flight data is quoted in this document.
Anyone who thinks otherwise has not worked in aviation and dealt with detail specifications for aircraft. Detail specs are ALWAYS the npreliminary estimates when the contract is dangled to the manufacturers. Sometimes the designers can't quite hit the points needed, many times they can. Sometimes the engine or finish or airframe fails to follow the planned parameters and they can't.
An excellent example is the Bell P-39. It did NOT hit the detail specs at all, though it got close in prototype form, if you believe the tests. Thye production P-39 was NOT a 400 mph aircraft except in a dive. In the prototype, engine did not deliver quite the expected power, the drag was not quite as expected, and the intercooler was not the low-drag, efficient unit that was needed. The bet result was the aircraft fell short of specs and the turbo was deleted, thus ensuring mediocre performance in the production versions.