does mikrotik webproxy support ip spoofing so the requested client ip address are pushed forward to the bandwidth management and not the mikrotik ip address itself?
no. you should use external squid proxy
oh ok thanks. i just somehow didn’t able to build tproxy + squid on ubuntu 8.04 for transparent proxy with ip spoofing. 
hm… from http://www.balabit.com/downloads/files/tproxy/README.txt :
Squid-3 has official support of TProxy v4.1
it’s not as easy as on the readme.txt. trust me. 
if it’s really here, the only problem I can see is with right routing of those proxy packets =)