It is very hard to compare superchargers in this way. Superchargers can be measured in airflow (pounds of air per minute), Pressure ratio ( how much they compress the incoming air) and efficiency (what percentage of the power used to drive the supercharger is actually compressing the air).
When looking at engine charts you are looking at a complete engine system that has to take into account the strength of the engine (blowing cylinders off is not a good thing), the fuel used (only so much boost can be used with certain fuels) AND the desired characteristics of the engine in question.
as an example the British Mercury radial could make 830hp for take off and 890hp at 1800 meters using a 7.0:1 gear drive on the supercharger (using about 914mm) , great for a flying boat or seaplane. Change the gear ratio to 9.4:1 and high level power went to 840hp at 4250 meters but take-off fell to 725hp. (using just over 1000mm) Same supercharger intake, impeller and housing/diffuser. Much better for a fighter or light bomber.
At 4250meters the Mercury supercharger was compressing the air 2.27 times and lets face it, the Mercury supercharger wasn't exactly the latest and greatest.
Just about anybody could design a supercharger with a pressure ratio of over 2 to 1 in the 30s and by the late 30s 2.3-2.8 was fairly common. The two big problems where getting the engine to stand up to it and getting fuel that allowed you to use a boost pressure of over 4-6lbs, 38-42 absolute or the metric equivalent.
Italian radial engines were rather light for their size ( in some cases very light) which leads to suspicions of a lack of structural strength to tolerate higher RPM or over boosting.
Lack of planning in 1939-34 (or later) is a bit understandable, the fuel situation changed dramatically. France was another country that got caught with engines that were fine with 80 octane fuel, OK with 87 octane but near hopeless with 100 octane or higher. The basic engines were not strong enough to stand up to higher boost pressures with out either major redesign or starting over.
German engines used 87 octane fuels too.