I installed SP1, and it did fix a few things, but it is hard to tell if it improved anything with HDMI audio.
The issues I had with the AMD driver were always sporadic, so I am not sure if they have been fixed or not. One is an issue that causes constant white noise when the receiver and TV are turned on (PC never sleeps). That happens about 1-2% of the time. The other is with popping in the speakers when I skip a chapter which doesn't always happen either. BTW, neither of those things ever happened with the Realtek driver.
Oddly enough, the two things that did seem to go away is launching a movie from Media Browser to be played in TMT5. It would always play immediately with BluRays, but required clicking play a 2nd time with DVDs. Well, that appears to be fixed now. The 2nd is with the Favorites plugin for Media Browser. The MC entry point would occasionally get messed up, and now that appears to be fixed as well. In the past I would have to delete the entry point and recreate it. It was broken before the upgrade, and after the upgrade it was working again without me doing anything else.
One last thing, maybe it is my imagination, but it seems as though the PC boots faster now. I didn't do any pre/post-SP1 timings to confirm this.