Turbomeca's S/C was the best part of the HS-12Y-45 (and -49), but these were just being introduced on the D.520s. The MS 405/406 were powered by the -31 engines with the old, legacy S/C made by the parent company (HS). Difference was big, talk ~200 HP more for the -45; power surplus was at all altitudes. The -45s S/C construction that employed variable guide vanes enabled it to have the power surplus also on the low altitudes (1-speed S/Cs that offer better hi-alt power usually sacrifice the low-altitude power).
(dashed lines are power, full lines are manifold pressure)
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H-S have had the -51 in the works, with crankshaft with ballances in order to turn extra 100 rpm, but it was a lo-ish altitude engine that made it's 1000 HP at some 3000 meters IIRC. Mating the -51 with the Turbomeca S/C would've provided a very good engine for the needs of 1940, if not for 1941, but even the -51 was too late in production to matter.
HS 404 was too late, even the French were still using the licence-manufactured cannons of Oerlikon design in 1940. Supplying the 404s to the E. European conutries was nigh impossible.
Oerlikon cannons were the best 20mm cannons in the world before 1939-ish, and were still very usable in any ww2 year - calling them indifferent is selling them short.
It took Klimov until 1941 to sort-out the HS 12Y, that included opting for smaller bore so the block is stronger, introduction of a 2-speed S/C, introduction of new valve train with 3 valves per cylinder.
Someone will have to pay to DuPont to make the factory to make TEL. Not impossible, but something else does not get made? Granted, a good/long credit (perhaps by Austrian bank(s) in mid 1930s?) can perhaps have the factory (factories?) running before things get pear-shaped, like German invasion of Poland.
Mercury VIII that runs on 100 oct gained some 150 HP at lower altitudes - that is a very good increase. Polish need to sort their fighters 1st, though - parasol fighters with fixed U/C have a problem past 1935.
