How much speed increase?: serial to parallel

[wily@svpal.svpal.org (Paul Creager)]

I finally freed up some slot space in my GS, and I'm debating getting a
parallel card for my HP DJ550C.  How much of a speed increase should I
expect compared to my 19200 serial connection?

Since parallel is moving 7-8 bits at a time, rather than 1 like serial, I
can't expect a 7-8x speed increase, can I?

Furthermore, any practical benefit to installing buffer RAM in my 550C?
It can take two 256K modules for a total of 512K.  I print simple text
stuff from P8 (ProTERM), and of course use the GS side for the fancy
stuff (Pointless, etc.).

Thanks,
Wily