Can it be guaranteed that it's going to be fast enough at high
baud-rates to prevent any gap between the first byte and subsequent
bytes?

Possibly not for some protocols (or worse yet 'almost always' for some
protocols).

It's something to look at once the basic bits are in.

I now work for a company that has manufactured PC serial boards for
25+ years, and we still get regular requests for that feature (and our
boards do support it -- though our Linux driver does not).

In which case when we get to addressing this it will be good to make
sure we cover your needs as well.

Alan