Good points, i've found out very similar stuff when first experimenting with it. My mnemonic rule for power changes is that "the prop should always be ahead of the throttle".
It takes a bit of practice but you can easily get the 109E to 500km/h at low level. I set both rads to half open (one turn of the water rad rotating handle, the oil rad is easier to set because it's a slider control in the cockpit and the interface info window), run 1.3-1.4 ata and reduce the RPM whenever i'm not climbing, just not too much (2100-2200 for cruise, 2300 for combat, 2500 for combat climbs).
The trick is to use high RPM when accelerating up to 300km/h or so, then reduce it so that the prop gets a bigger "bite" of air. This is done in steps and to judge the RPM i use the engine sound and the little clock-like instrument because it's easily visible without having to look lower into the cockpit.
I don't have a spare slider for prop pitch so i use two buttons on the base of my sidewinder precision pro 2 to increase/decrease it. This means i don't get completely smooth control (it moves in preset notches), but it's possible to maintain intermediate settings by alternating the button presses (although it's not really necessary).
I start with pitch at the 12:00 position, then reduce it to 11:30 once i reach 300km/h, i then reduce it even further in two more steps down to the 10:10 position.
This will easily give you 500km/h at full throttle in level flight and about 400-450km/h at a combat cruise setting of 1.2 Ata. Just remember to increase it when you want to climb, it's even better to anticipate your climbs and increase the RPM beforehand because it takes a bit of time to change.
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