That's difficult to do on the ATM switch, and not really necessary since we
can measure IP packet size any number of places and compute the ATM overhead.
I have been doing some more arithmetic on the "ATM cell tax" to see if I can
find out how folks get from 10% (which is the 5/53 overhead) to 35%
overhead. I would really really like to see some backup for those sort of
numbers -- I can't make the arithmetic work out to anything near 35% for
what I think are useful assumptions. Of course, if you want to posit 100%
of IP traffic with 49 bytes, but that doesn't seem realistic and you
shouldn't forget that 40 of those are TCP/IP overhead. 
What traffic mix do you use to calculate ATM overhead and what results do
you get?
--Kent