Yes, but...
1) You will need hardware flow control as the 6551 only has a one byte
input buffer.
2) My Rockwell manual does not show this but I seem to recall that
some (all?) 6551 chips will corrupt the byte currently being sent if
CTS is deasserted mid-character - the 65C51 does not have this
problem.
3) You really need an accelerator card/chip to take advantage of the
higher throughput.
As usual, you made the assumption that I implied that
I would want my modem set to 115,200 baud, and I did
not. If I had meant that, I would have said, "to these
settings". ;-) I try to say what I mean, and mean
what I say, and have come to the conclusion that people
don't hear correctly. ;-)