The rollrate of the 190 is based on
non linear optimization of wing aerodynamics. Of course WW2-era planes were NOT unstable - else the pilots would have a very low life expectancy (30 seconds after their first take off, more or less).
Obviously an extreme reaction to any minimal moving of the commands is the upsidedowned effect and fighters are intrinsecally instable, the more fighters are instable the more they are aerobatic planes. This last part is quite noticable in Aces High while flying various planes edging the stall speed in a knife fight.
The limit is the fatigue exerted by the pilot and the correct balance was an artistic mix of allowed instability vs. stability.
Modern jets are all super-instable because the cloche is directly governed by the computer "buffering" all the corrections needed to go in a fixed straight direction. While the pilot imposes thru a joy-stick the correct changes of direction will not be send to the wings but to the computer.
The real pilot is a super-reactive computer but the 1st in command is always the human pilot. Also the reactions of the stick are "simulated" and regulated following the taste of the pilot himself.
Heh, I'm not at all sure about that. It seems that they could make the Spitty roll as fast with a much bigger wing by simply clipping it wingtips off... 
So, they clipped the wings on the Spitfires to obtain better roll-rate but it came at a price, It hindered its high-altitude performance because of reduced lift and increased drag.
EDIT: I found a comparison document about the F6F-5 and the F4U-1 vs the Fw 190:
http://www.wwiiaircraftperformance.org/fw190/ptr-1107.pdfBottom line: I think i'd better do research myself before i start posting
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