On the first point, it may be relatively easy to test, by running a small 3D app in a window to push the clocks up, and seeing if the desktop responds better moving between screens.
As for the second, there are three reasons I know of that the GPU will reduce clock speed:
1) Reduce temperature below the target. Not an issue in your case.
2) Reduce power consumption to fit into the envelope. May or may not be the issue in your case.
3) Reduce power consumption when the load doesn't warrant it, due to the CPU not keeping up.
You suggest it only happens in games that are less demanding, which lends support to the third option. If the CPU is only able to set up X frames per second to render, the card will only run as fast as it needs to in order to match that rate. With previous Radeon models, there were only three power levels, so it wasn't often that you'd see the card downclock under low load. With the current crop, they can reduce clock speed by an essentially arbitrary amount, so you'd expect them to downclock much more frequently.
It's possible, of course, that they do so too aggressively, with the clock changes unable to respond to changing demand fast enough. Some posted complaints support that possibility. Perhaps there's a way to lock clock speed and run some tests to see, but I don't know off hand if it's possible.