I added some brief statistic code to tty_subr.c to get an idea of
how q_to_b gets used.  On a moderately busy system with ~10 users,
an active ppp line, web server, etc, q_to_b gets called thus:

56023 calls with an empty q
181044 calls with data
129 bytes in the q on average

not suprisingly, the q size goes up with heavy ppp use 
(we're on a v.34 modem)

-j