Urchin, see it all becomes a matter of pilot skill (not really sure skill.. maybe more knowing what he's going to do before he does it), plane type and fuel load in those engagements.
A P47D-25 with only 50% in his main tank while 0% gas in his aft tank will without a doubt be able to get onto a 109G2's tail and stay there if the 109G2 has 75% in his main tank. Those are about equal fuel load weights between the two, what it comes down to is wing area at low speeds. Something the D-25 has more of than the G2. The 109F4 though is no contest it will undoubtedly turn inside any American plane currently modelled regardless of fuel loads.
I believe that the 190D9 and 190A8 also turn about the same, maybe the D9 is a little heavier on the controls.. BUT the 190D9 has the engine power and ability to regain energy quicker than the 190A8 which in turn means more chances to make mistakes and still regain his energy to continue the fight. The 190D9 will lose a turning fight against any currently modelled American plane though, but it has the speed to escape.
For the 109G6 it's really a sluggish turner at low speeds. It's also a toss up on fuel load and pilot ability. In a pure sustained turn, a Jug or P51 with light fuel and a notch or two of flaps can get inside the G6, especially at low speeds. But the higher the speed, the better the chance the G6 has of getting inside. Here's the kicker though, you CAN use a spiral climb sustained turn in the 109G6 and the P51/P47/P38 will not be able to get inside because the G6 will simply climb away while those planes are sitting back there struggling to get enough air over their elevators to maintain their turn.
One thing I am certain of though, is that the 190A5 vs D-25/F4U-1D(or C)/P51D(orB)/P38L it all comes down to pilot ability and fuel loads. Typically if you are up against an average pilot in one of those american planes, sustained turn the 190A5 will easily win out. For example, several tours back, I was dogfighting a P51D and F4U-1C. I managed to keep inside of the F4U-1C and still be able to turn away from the P51D(he kept making long drawn out passes). I managed to hang on the F4U-1C's tail long enough to shoot off his entire tail section using just the 7.92mm nose MGs. (400 odd rounds) The P51D I managed to get to commit to the fight (by now I had about 60% in my main tank, 0% in my aft) and when the P51D got slow I simply stayed on his tail no trouble and emptied about 20 cannon rounds into his wing and he crashed. The P51D pilot failed to drop one or two notches of flaps though, so this could of easily been the reason for me to stay on his tail so easily.
Here's what I suggest you do, if you want better research than simply going to the DA.. I find this method helps to understand all aircraft better: Go offline (yes I know it's boring ;-) and test the various aircraft you think might be a close turner with your plane.
Get a stop watch and a pad of paper and pen. Take off and allow your plane to get to a set speed (250MPH to start with) and have your compass stay with the heading "North", bank left or right (try both ways, depending on engine torque you might be able to get a faster turn in one direction) and begin turning as hard as you can without stalling (be sure to start the stopwatch right when you begin turning), when you are facing north again, stop the stopwatch and record the time, speed, fuel and flap settings.
Repeat for each plane trying different combinations. I was amazed at what the American planes can do with a notch of flaps (something the 190 can't do because they don't deply until around 160MPH).
-SW
