After a few hours digging around, I eventually figured out where the relevant sockets are created and have added a patch (to 7.4-stable and -current) to bump buffers on them.
- 7.3: Squid 6.3 with multiple workers - net.unix.dgram.se... Silamael Darkomen
- Re: 7.3: Squid 6.3 with multiple workers - net.unix... Claudio Jeker
- Re: 7.3: Squid 6.3 with multiple workers - net.unix... Stuart Henderson
- Re: 7.3: Squid 6.3 with multiple workers - net.... Stuart Henderson
- Re: 7.3: Squid 6.3 with multiple workers - ... Stuart Henderson
- Re: 7.3: Squid 6.3 with multiple worker... Silamael Darkomen
- Re: 7.3: Squid 6.3 with multiple w... Stuart Henderson