How long has the 5.5 server been operational ?
If it is a newly built server, it's squid cache and dns cache will be very small, so very little performace increase will be seen.
How many users are there ? - It may be worthwhile increasing the number of squid child processes.
I think that 5.1.2 was optional to use squid.
5.5 uses squid in transparent mode ( ie even if no settings in IE/Netscape, it will still route traffic for port 80 via squid. )