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Dean,
In my opinion (and fairly well technically verified) there is more Sysopmania
and noise to the 16550 (as pertains to most Fidonet *_sysops_*) then the
benefits claimed in some unrealistic and error filled messages I have seen
...  ie little, if any, improvement for most will 'REALLY' be observed (if
accurately measured).  Yes, I know ....  this is contrary to the almost
endless stream of those who have heard the word, now supposedly see the
light, become believers, etc.  This is after all an electronic religion and
Gods, self-appinted, elected, or otherwise, do exist ....  and since they
have spoken ... the believers (must) will replicate the 'word'.
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What is a 16550?  What does it do?  Is it a new 'super' serial chip?
A 16550 (as the 16450 & 8250) is a UART - Universal Asynchronous
Receiver/Transmitter chip.
It is used to interface the Computer with serial devices (eg a modem),
controlling data sent back and forth, based on inputs from both the Computer
and the serial peripheral (modem in your case).
The 8250 was the chip originally used in the PC & PC XT systems.  People
still using this UART will TEND to have problems if communications at higher
than 9600 are attempted (ie. 19200 or 38400).
The 16450 UART is widely used in the AT & 386 class machines sold today.
This chip can almost always handle the 19200 locked com port speeds of
currently available high speed modems.  Theoretically it is possible to still
sometimes encounter a overrun even with a 16450 (although extremely
rarely.... and that other factors even 'more significant' then the UART
exist).  TSR programs/hardware are frequently HEAVILY interrupt driven (ie.
DESQview, PERSTOR controller).
The 16550 is the same chip as the 16450, but contains a 16-byte FIFO
(First-In, First-Out) buffering queue ....  four-bytes greater. This buffer
is SOFTWARE controlled  (ie.  QModem, OPUS!Comm v5.30, X00 v1.10x, BNU). This
allows the chip to be  able to receive/send data even while the CPU is
servicing an Interrupt. This helps keep the system from losing Interrupts
(and lost characters).
The 16550, 16450 and 8250 are normally in a 40-pin DIP package that are
plug-compatible. There are also SMD versions of these devices (common in
PS/2s). SMD refers to Surface-Mount-Devices. Allows more compactness in
design, but pain when a chip goes bad....
The contribution offered by a 16550 over the 16450 is minimal .... when
compared to other factors and their solutions.  If one still has an old I/O
Comm card with a 8250 and a system speed of 12MHz, or higher, ... they would
realize a benefit by replaceing the 8250 with a 16450 or 16550 ..... but if
one already has a 16450 and 25MHz or slower system .. little, if anything, is
gained with swapping 16450 for 16550.  Yes, if one reads echomail claims of
speeds in excess of almost anything exists (the mouth artists).  But these
are not facts or 'acurate' measured data.
                                                                   FYI !