That's nearly a character every uS.
Rob, it's late in the evening and I'm sitting here on my couch with a nice brandy - just right atmosphere for hair splitting. 921600 is only 1 bit every µs, so he'll have 10µs per character. But that doesn't really change matters and all you wrote about the timing being tight and forgetting all those soft serials are still true.
However, if he isn't receiving data but mostly sending, the whole thing will probably just work fine. One doesn't need to send at full speed.