I had a chance to get some good practice with FSD 12.3.3 (or whatever it is) on some rural roads during a trip last week. These were a good test bed because there was little traffic, and there were some good straight sections in addition to curvy sections so I could observe its behavior. Also worth noting is this was in rural New York State where many times when you come out of a town, you just see a speed limit sign that say something like "END SPEED LIMIT 30", at which point you are to assume that the state speed limit of 55 takes over. In other words, FSD has no clue what the speed limit is, and assumes it's still 30.
So first of all, yes, FSD 12 notoriously drives below the speed limit (whether automatic speed is set or not), but usually it will get at least close to the set speed if automatic speed is off). I've had to nudge it with the accelerator sometimes, but if you are patient and the road is straight, it will eventually get there on its own.
But with our without auto speed set, the car basically responds to nudges on the accelerator pedal to indicate you want it to go faster, and it will generally respond and maintain the new speed you've achieved by doing this.
Now I did want to see how the car behaved on those rural roads where it thinks the speed limit is 30. Remember, these are clearly 55 roads (and people generally drive 65 or so on them since they are straight for long sections, with little to no traffic). So I set I was pleasantly surprised to find that FSD did not hesitate to exceed what it thought the speed limit was (based on what its little speed limit sign was showing). It even exceeded the speed limit at times without prompting (very curiously). In fact, at one point it was doing 67 along a road, and I usually like to keep it under 10 over. I wonder if they've been collecting data on how fast vehicles actually drive on those roads and use that as input to the "natural speed" used by the auto speed feature.