Maxim,

Thanks so much for clarifying. Just to make sure I'm understanding correctly, if I had something like this pseudo-code

upstream upstream1 { }
upstream upstream2 { }
upstream upstream3 { }
upstream upstream4 { }
upstream upstream5 { }

server { server_name server1.com; proxy_pass http://upstream1; }
server { server_name server2.com; proxy_pass http://upstream2; }
server { server_name server3.com; proxy_pass http://upstream3; }
server { server_name server4.com; proxy_pass http://upstream4; }
server { server_name server5.com; proxy_pass http://upstream5; }

There would only be performance degradation if the setup was:

server {
server_name server6.com;
set $PROXY_TO 'upstream5';
proxy_pass http://$PROXY_TO;
}

Is that correct? And, if there was degradation, would it be limited to hosts that server block was trying to serve or would it impact overall performance?

Thanks!