>> Try building a current kernel with options COM_16650 (COM16650 on 2.0).
>
> Well, this is interesting.  I saw that option in the sources, but my
> eyes misled me to read it as 16550 and I just assumed it was on.
>
> I didn't even know there was such a thing as a 16650.
>
> And, indeed, with that option, the card probes as a st16650a
>
> However, cu still doesn't accept -s >115200, nor does stty.
> EINVAL from tcsetattr.

what is your card?

is it ISA plug-in? then NetBSD don't know it's base frequency so it 
assumes it's 16*115200 and max speed 115200

if you know it's max speed is say 4 times 115200, just divide speed you 
need by 4