Further thoughts
What if the BF 109F4 actually achieved 670 Km/h? Shouldn´t the G2 be faster due to a more powerful engine?
Perhaps. Compare:
Kurfürst - Mtt. AG. Datenblatt, Me 109 G - 1. Ausführung
The test results show a max. speed of 632-664 Km/h (depending on individual aircraft) with:
radiator flaps 120mm open, while the usual opening was 40mm at "Schnellflugstellung"
but more seriously:
The DB 605 wasn´t cleared for WEP, so this speed also only is for "Kampf- und Steigleistung" -powersetting
This is beeing confirmed in the latter part, where the G1 achieved 660 Km/h at 7000m altitude. But here again the engine wasn´t cleared for WEP, and thus those climb- and speed figures are attained on normal, 30min. sustainable 100% powersetting.
Factoring this into comparison, the G1 and G2 indeed were faster than the F4 at the same 100% powersetting (G2: 660 Km/h; F4: 635 Km/h).
But how fast could they go with WEP?
The difference is striking for the DB 605:
at 100%: 1310 hp @ 0ft.
with WEP: 1450 hp @ 0 ft.
Since the airframe is the same and all other factors beeing equal, the 10.687% power increase should lead to a 2.67% speed increase (You need four times the power to double the speed under lab condition), or in other words, the same G1 testplane should be able to go 678 Km/h (421 mp/h) at 7000m altitude for brief times using WEP (Bf 109G1).
Now back to our Bf109 F4 with DB 601E and 635 Km/h at 100% and 6400m / (640 Km/h at 6700m) altitude*):
at 100%: 1200 hp @ 0ft.
at WEP: 1350 hp @ 0ft.
The 12.5% increase in power should translate into a 3.125% increase in speed at our reference altitudes for WEP, ergo the same Bf109F4 testplane should be able to go 655 Km/h at 6400m or 660 Km/h (410 mp/h) at 6700m using WEP powersetting (Start- und Notleistung).
I conclude that the 670 Km/h figure is off by 1.5% from the expected one.
Perhaps an individual plane in excellent state may go that fast but it cannot be expected that all production models achieve this speed under the given circumstances. My thought.
Since the BAL limits for production models were 3% (+-), this is in within acceptable tolerances.
*)
Kurfürst - Mtt. AG. Datenblatt, Me 109 G - 1. Ausführung
the dataset shows 660 Km/h @ 6200m altitudes for 100% (Kampf- und Steigleistung) powersetting for the BF 109F4 (not G1 as the link name imply)which does not agree to the 635 Km/h attained by other tests. The explenation may be that this specific test -unlike others- wasn´t recalculated for compressibility effects, as the author suggest (check notes). Confirmation comes from OKL Typenblätter, as this dataset was not used furtherly for entry into Typenblatt.