Need to increase nginx throughput to an upstream unix socket -- linux kernel tuning?
It sounds like the bottleneck is the app powering the socket rather than it being Nginx itself. We see this a lot with PHP when used with sockets versus a TCP/IP connection. In our case, PHP bottlenecks much earlier than Nginx ever would though.
Have you checked over the sysctl.conf connection tracking limit, socket backlog limit
net.core.somaxconnnet.core.netdev_max_backlog
You might try looking at unix_dgram_qlen, see proc docs. Although this may compound the problem by pointing more in the queue? You'll have to look (netstat -x...)