The stall horn will begin to sound at different speeds and at different turn rates in different planes. The stall horn doesn't sound at various turn rates in the same plane. Either you're entering the stall or not and it's more speed than turn dependent. It just happens the harder you turn the slower you go and the more likely you are to enter a stall. What I'm saying is that when the stall horn just begins to sound in the plane you're in you're likely at your best sustained turn rate. That's much different in a 190A-8 than it is in an A6M2.
Instantaneous turn rate is a whole different topic and generally happens well above stall speeds.
The stall buzzer actually does sound at different turn rates (and speeds)
in the same plane. All the stall horn is telling you is that you're near the lift limit, that's it. It doesn't care what speed you're at, or your turn rate, or your altitude, or G, just the critical AoA. If you're at the lift limit at 350mph you'll get a certain G which will give you a certain turn rate and radius. That's the best
instantaneous turn rate/radius at that altitude, loading and speed but you can't sustain it in a level turn due to insufficient power.
If you hit the lift limit at 200mph your best instantaneous turn rate/radius will be different (lower rate, larger radius) than at 350mph due to less G available and, as 200mph will likely be above your P
s=0 line, you can't sustain that either. Your rate/radius will continue to change until you decelerate to the intersection of the lift limit and P
s=0 line at which time you can sustain a level turn at the lift limit without either climbing or descending but at even lower G than in the previous examples. This is the point where you get the aircraft's best
sustained turn performance.
Note that those very different parameters (speed, turn rates, and turn radius') occur while the stall buzzer is going. You can also see that instantaneous and sustained turn rates aren't really different topics, they're the same thing just at different parts of the flight envelope. The difference between "instantaneous" and "sustained" is simply a function of power available and logically you can see that if you're willing to trade away altitude you can improve your sustained turn performance for a short while (Terra firma being the limiter). It's also obvious why a spiral climb reduces your sustained turn performance.
Hope this clarifies things a bit.
