How sensitive is the air-fuel ratio to fuel level changes? If I raised mine from .400" to .300", it would come onto the main circuits sooner, but how much of a change to the AFR -- assuming there is a change -- could be expected?

If youre talking about in the bowls, for sure matters. Can't say that a .1 change is enough to notice anywhere except the dyno though. I've gone from top of the sight glass, to below the bottom,.5 and saw enough change to effect mph. With all the sloshing, g forces, and float bounce that goes on in a normal carb don't think it's a way to tune.

I have a few sets of bowls wj used when ps was on carbs. The floats have hydro shock absorbers and tons of work in them to control the fuel level through the run, and they just stuck with one position for the floats.

My experience has been, unless the floats are just “too low” or “too high” for proper carb function, it is usually more of a “fine tune” to the fuel curve on the dyno.
However, I’ve seen it where the best power came from a float position that just didn’t work out well with the motor in the vehicle.

It is one way to change the tune on the dyno but for a street car I'd put the level in the middle of the glass and leave it alone.

An even quicker way to change the tune on the dyno is to use the idle feeds. The idle feeds will give you a full point of AFR. We got in the habit of just turning the idle feed screws if the engine was within a point of where it needed to be. Much quicker than opening up the carb and changing the jets.

Once again, I don't think I'd tune a street carb with the idle jets but it can be a fast way to try something.

Not quite an apples to apples comparison, but when we made the switch here to e10, a lot of small engines had lean issues. Raising the float levels has an effect on the emulsion tubes, so a quick tweek to the float level would(and still does) make them run nicer. I would imagine the same effect would apply to a Holley.

the best power came from a float position that just didn’t work out well with the motor in the vehicle

Or when the G force acts on it and the gas tries to climb the back wall.

A change in fuel level would affect the time it takes for the fuel to reach the booster. The closer it is to the booster the sooner it will begin as well as where in your fuel curve your emulsion begins

You would have to test it with an 02 or 5 gas to be certain.