I just got the official word (from SanMehat in #android - thanks!) that the tethering speed problem (80k/sec or, coincidentally, 115200) requires an upstream fix (in-progress). The current driver is only capable of the basic 115200 speed. However...
"we have a new driver for the High speed UART but its not quite ready yet"
And the general feeling at google (from SanMehat and jbq and others) is that they are "really looking forward to working closer with the community now that the dev phones are available."