Speaking of the 12.3 highway stack..
While I haven't been on the highway with 12.3.2.1, I have been with 12.3. For what it's worth, I've been driving around with the Auto speed set. I understand the complaints from others; but I've been finding that, while it runs a bit slower sometimes on local 2 and 4 lane roads, it often runs faster in certain circumstances. For example, there's a striped 2-lane road whose speed limit is 25 mph; with both 12.3.2.1 the car sort of vaguely wobbles (depending upon what other cars are doing and its own ideas between 20 and 30, with the slower in congested areas and the 30 in places less so. Interestingly, sometimes, after turning a corner at, say, a red light that turned green, in a 25 mph zone the car will zip up north of 30, then gradually come back to 28. Slightly weird - but not unsafe, and not overly speedy.
Which brings us back to the highway stack. So, on 11.4.9 I didn't have any offset dialed in; when getting onto a highway that was, say, 65 mph, it would speed up to 65 mph. If I wanted something a little faster (not at all unusual), well, that's what the right scroll wheel was for, and I'd put it up to 70. Or whatever felt comfortable to me.
With 12.3, with auto set, as the car would enter a 65 mph zone, it'd put the speed limit to 72 mph. Every time. Now, I could adjust it up or down from there, 11.x style, but 72 is where it always went.
I double-checked it: No, I don't have an offset dialed in on the highway stack. Apparently, the initial set point is set by the auto function; once it lets one loose, it runs from there. Further, there's one section of road where the speed limit is 55, then upped to 65 a few hundred yards up. It would stay with 55 - and then zip its setpoint up to 72 as the 65 mph sign was passed.
Now, admittedly, people in the far left lane are kind of going 72. But I suspect that there's some, "Typical speed for this highway" buried in the map data somewhere.