For anyone who is interested in nginx tuning, please follow the H5BP nginx repo: https://github.com/h5bp/server-configs-nginx, which is very well documented already and still being maintained.
It's a shame there isn't one for reverse (HTTP) proxies: fine tuning proxy settings: proxy_buffers, buffering, etc, and other related settings when dealing with a back end application.
The OP recommends turning off gzip in MSIE6 when it was only the very first versions of IE6 that had a problem with gzip and it was fixed in later versions.
And IE6 users are known far and wide for how fervently they upgrade. ;-)
[1] Why `gzip_disable` is added: https://github.com/h5bp/server-configs/pull/92
[2] Why `gzip_disable` is removed: https://github.com/h5bp/server-configs/issues/145