I don't think anyone has the luxury of manipulating numbers to their own objectives, when your whole country is on the brink of destruction. Nowadays even a kid can go look up a good book and think "hmm.. was Germany stupid? They were fighting two fronts, and it's plain stupidity. They can never win!"

, however, this strategical analysis may not have been so convincing when the German military is "currently" marching to Crimea, Ukraine and started to surround Leningrad and Stalingrad.
Thus, I think use of data in the sense of propaganda and military use, should be differentiated. In that sense, I think Hohun's right - they would spew all sorts of fantasies to the public, or even send up 'optimistic' reports to their superiors - but the people directly in charge of departments concerning intelligence, R&D, and aircraft construction probably all knew the truth, and made adequate decisions and developments according to it. ('boss guys' are always clueless, whether be it the Soviet system or the capitalist one

the 'Dilbert' irony of it!)
Frankly, I think the 'false data' accusations is about as equal as the over claimings of planes shot down and destroyed by bomber gunners of the West. In the 'inner circle', there's always a different truth to tell.