hi ccc,
IMHO - if we are talking about stability - we are not just talking about the application itself, especially networking applications. probably a comparison of 60-65% hardware performance and 40-35% the application itself.
the application (such as squid) representing software features - its ability to do certain functions - its tunable parameters, but will not meet againts hardware quality/capacity.
if you already satisfied with squid 2.6 (its features met your requirements) performance - i think that will be enough. otherwise, feel free to upgrade
just my opinion,
HTH.