Local ex-PIC rep claims to have a customer with 5 Million or so 16C57's
    running at 20 MHz AND 3.3 volts.

Using the built-in oscillator?  Merely "running" at 20MHz might be
considerably easier than driving your crystal at 20MHz.  (which also implies
that you might be able to get by with replacing the crystal with an
oscillator, if current consumption isn't a big issue...)

BillW