The 9.9.2 update now appears to not freeze up FCP X in mavericks, but there still is a long lag from pressing play, and wen the video starts. If audio and video aren't sent out to my UltraStudio Express, and just play and listened to natively on the iMac (2012) then playback is instantaneous. But send video out to the Express, and you get a half second delay. If you also send out the audio (via the control panel - appears the only way to get FCP X timeline audio out to the express) then the delay is closer to 1 second.
Another issue still remains. If you send interface sounds effects out, there is a HUGE delay if you hear them at all. Notably the volume up/down sfx. AND when BMD sound out is enabled, the volume sfx (and only the volume sfx) are forced to go out the express. Even if you tell sfx to go out the headphones of the iMac. And when the volume sfx go out the express, then they have a huge delay or only sound every 4th click or so. Other sfx like alerts and mail act correctly.
Nice that it doesn't lock up FCP X and a timeline can now play out, but the lag is still pretty annoying as well as the volume sfx issue. Not sure how much of this is mavericks and how much is BMD.
Lets get some Speed Grade output support while we're at it guys!