Bf109E-3

Just as a side note: 5% of 500 kmh = 25 kmh, so a 475 kmh fast Emil would still have been accepted

Just wait and take a look again when mixture ratio is fixed. This might in fact lead to more nominal values when finally this parameter can be optimized.

When talking about drag one has to be a bit cautious when saying which airframe is less draggy when talking about flight operations as drag is composed of friction drag and induced drag. The former is due to friction over the skin and therefore mainly dependent on speed and altitude, the latter foremost dependent on angle of attack. So it might be (might be, I don't say that it should be but yet could be) that the spit just had to fly at an angle of attack at high altitude that went along with higher induced drag while it had to use less angle of attack at lower altitudes and therefore had less drag. Actually it is quite reasonable to assume that this is indeed the case. When the air density decreases the plane has to increase angle of attack for level flight but then can reduce angle of attack with tanks more empty. The behaviour could be very much different for the 109 and the Spit. I cannot tell in which ways.