I think there are a few more things that need to be addressed.
Valve overlap and multipoint fuel injection can get 10% to 15% more power.
The DB 601 already used multipoint fuel injection so using it on the DB605 shouldn't really be responsible for any increase in power.
The Daimler Benz engine seems to have been more fuel efficient, suggesting a smaller radiator as well.
While the Daimler Benz engines were more fuel efficient jumping to the smaller radiators is a bit of a stretch. Maybe they were smaller but radiators also have to matched to the aircraft. A bomber using the same engine as a fighter might very well have to use larger radiators because of the more extended but slower flight speed high power periods of flight. Like a long slow climb to operating altitude. Engines also wound up with different amounts of heat rejection to their cooling and oil systems. An Allison and a Merlin of about the same power loose different percentages of heat to their oil system and cooling system from each other. At full power (or close to it) extra fuel (part of the rich condition) was used as an internal coolant.
Again the boost level of 9 psi is equal to 1 ata plus 9/14 = 1.65, a level not introuced (1.7 ata) early 1944 on the DB engine.
This smacks of making a virtue out of a necessity. Didn't the Germans have some initial problems using even 1.42 ata? like burned piston crowns and things? Trying to move to 1.7 ata without better fuel or charge cooling is just going to make things worse.
The technology the DB605 used simply didn't need high boost levels nor did it need inter cooling.
The supercharger was simply less importation a factor to this engine.
There was no 'secret' or 'special' technology. It was a series of choices that were made that were know to all the major aircraft engine companies. In the 1930s with 80-87 octane fuel and for a given engine weight you can make a smaller, high revving engine (Merlin/Allison) or you can make a larger displacement, slower revving engine. The available fuel simply wouldn't allow much boost to be used and the detonation limits required trade-offs between cylinder compression and boost levels. Many engines used compression ratios of 6.5-7.0:1 but low boost(usually under 5lbs or so). Point Fuel Injection was known and experimented with by several companies, it was also expensive to make and maintain and some countries, to be honest, just didn't have the infrastructure (number of skilled subcontractors) to support it's adoption at the time. But the engine designers did have a pretty good idea of it's benefits. The negative "G" thing did kind of slip by them though.
At sea level 75 in of manifold pressure (22.5lbs boost) or 2.5Ata is just 2.5 times the surrounding air pressure and can be achieved rather easily by most single stage compressors. The ability of the fuel to allow it or the engine to stand up to it were the limiting factors. The problem comes with altitude. An intake pressure of 54in (1.8Ata) at 6000meters needs a pressure ratio of 3.92 this was much harder for a single stage compressor and was frankly NOT possible before or during the early war years. An intake pressure of 60 in (15lbs/2.0Ata) at 23,500ft (Merlin 61) needed a pressure ratio of 5.1 to one and an intake pressure of 44 in (7lbs/1.46Ata) at 25,000ft needed 3.96 (early P-38).
By Not possible I mean getting that pressure ratio in a useable fashion. Not only do superchargers have pressure ratios but they have efficiency ratios. Usually for aircraft they tried to keep the efficiency up to .70 or above. Some of the late 1930s superchargers could NOT provide a pressure ratio of even 2.8 with an efficiency of over .70 .
The efficiency is how much power is actually used to compress the air. If a supercharger took 100HP to give desired flow (mass and pressure) at .70 it means that 70HP is going to the work of compressing the air and the rest is going into the the friction in the supercharger drive and bearings AND INTO HEATING THE INTAKE CHARGE. Since the mechanical loses are only a few percent (at worst) that means almost 30HP is heating the intake air. Raising the pressure ratio at the expense of lower efficiency means a less dense intake charge (fewer lbs of air per cubic foot) and a hotter intake charge pushes the detonation limit. It also takes more power to drive the supercharger. Supercharger pressure ratio ( for a given supercharger) is fairly proportional to the impeller tip speed and power required goes up with the square of the tip speed. Changing from a 7.0 gear ratio to a 10.0 gear ratio doubles the power used by the supercharger. It also doubles the amount of heat absorbed by the intake charge over and above the heat of compression even if the efficiency of the supercharger stayed the same and they usually did not. Efficiency dropped a few points at the higher pressure ratios.
This why the push came for two stage superchargers in the pre- war and early war years. The existing single stage superchargers could not provide the altitude performance that was desired.
The DB605L for the almost in service Me 109K-14 (and DB603LA) had two stage superchargers but did not need to use inter cooling
But they did use MW-50 much like the 2 stage Allisons in the P-63 used ADI instead of an intercooler.