The noise is slightly present at all speeds, and given that it is a convertible, with all of the other noises, it is not an issue to me. However it becomes really "bad" at about 40 mph. That is, anyone sitting in the car would probably ask about the noise. Easing up on the accelerator or completely taking my foot off of it makes the noise go away or diminish back to acceptable levels. At much higher than 40 mph the noise seems to lessen, but that is tough to tell since the windows are open and like I said, in a convertible, you get to hear a lot of outside noises, other cars and trucks, etc. I have had the car up to 75 mph on the highway for short periods of time and it runs very well, the ride is stable, no vibrations and all you really hear is the road and air noise. So the axle whine does not continue to increase, and it may actually be decreasing, but like it said, tough to tell.
I have not noticed a difference going around curves. It certainly decreases or almost goes away in coasting mode and increases in acceleration.
Axle oil is 85-140 synthetic.
Checking all of the tolerances could be a course of action before a major project like replacing the ring and pinion. But I certainly want to avoid a replacement gear set that is not Ford NOS.