Well if turn rate is not proportional to Gs, that is not what this source seems to says: It is from a Kirtland air force base military magazine (link doesn't seem to work for me though):
http://safety.kirtland.af.mil/magazi...8/pullout.html
"Take me at my word when I tell you that turn radius is proportional to the square of the airspeed and inversely proportional to the number of Gs pulled [R V2/n]. When the airspeed goes up, the turn radius increases very quickly if we hold Gs constant. Hold the airspeed constant and the turn radius decreases when the load factor goes up.
Contrast this to the relationships for turn rate. Turn rate is inversely proportional to airspeed [* n/V] and proportional to Gs. As you go faster, turn rate decreases. Conversely, the turn rate increases as you pull more on the pole. "
I always assumed the "Corner Speed" was the lowest speed to pull maximum structurally safe Gs, and so
also the point of the highest possible rate of turn, since it is also the point of the smallest radius at this uppermost G level...
Since lower speeds means a smaller radius but also lower Gs, that should also mean a lower turn rate, as you cannot pull as many Gs below Corner Speed.
If you say Gs and turn rate are not proportional, then that would mean a higher turn rate than "Corner Speed" is available below Corner Speed, which doesn't make sense to me.
Basically, my understanding of sustained turns was that they should be done as close as possible to the "Corner Speed", while knowing well that in WWII that meant sustaining low-speed turns as fast as possible,
with as much power as possible, given that the structural Corner Speed limit was
way above in Gs what those weak WWII engines could keep the speed constant at...
A gap of about 50-100 mph + it seems, from 200 sustained to the 250-300+ MPH lowest speed to reach 6Gs...
The "Fighter formation qualification program", in "Fighter formation fundammentals", also seems to agree:
"Corner Speed is the airspeed at which the highest bank angle can be achieved at the minimum airspeed. Thus maximum rate of turn will be realized at this speed."