I agree that 190 was a fighter and 109 a sports plane with guns (much like Zero). As to the MW50 and why Allies did not require it...
A litre of gasoline contains the same amount of energy, no matter what the octane rating. However, high-octane gasoline takes higher pressure and temperature to ignite. German synthetic fuel (that they were forced to use late in the war) was commonly only about ~85 octanes (roughly comparable to 80/87 rating) or even less: Allied engines commonly used 100+ octane fuel, with some extreme examples like P-38 Lightning requiring 130+ octane (115/145) fuel to run properly.
What happens is that if you raise the boost pressure too high for low-octane fuel, it will prematurely detonate and actually use part of its energy working against the movement of the piston, a phenomenon called "knocking". The lower the octane rating, the earlier the detonation and the more energy is wasted and converted into extra heat. The MW50 (50/50 mix of water and ethanol) was used to overcome this temporarily by cooling the engine and thus delaying the detonation, so that high boost pressure could be used (temporarily) with the low-octane synthetic fuel. And here's the kicker: MW50 was only effective up to about 6 klicks of altitude, after which it starts to lose its edge. Now where have I heard that before, surely that is not the altitude at which a certain fighter started to lose its performance?
There was an alternative, the GM-1, which car tuners of today know as NOS, ie. liquified nitrous oxide, which instead increases the amount of oxygen in the fuel, acting more like an "extra turbo", and that worked at any altitude and for all octane ratings. Which is why it was used in Ta-152H (high-altitude interceptor variant of 190, hint hint) alongside with the MW-50. The Ta-152 does also prove that the 190 design still had much more development potential left than 109. However, GM-1 had negligible effect in low altitude compared to MW-50, and it was really complex to install dual system, and when not in use, both added significant weight.
What this means is that Allied engines were designed for continuous higher boost without the need for extra gimmicks because they had high-octane fuel available in large enough quantities.
So if you ever wonder why IL-2 Sturmovik (the game) has underperforming American (and to lesser extent, British) fighters, especially for the late-war designs: dear Oleg only trusts "neutral" sources, in this instance, German (and in some cases, Soviet) test flight reports of captured American planes. Which obviously had very poor performance when running on wrong kind of fuel, but since factory test flight reports and first-hand experiences by American pilots are all untrustworthy propaganda according to Oleg & co...
