We have just just upgraded our server to 1.4.1 and implemented some of the logging features. For some time now we have been running your suggested OS tuning settings.
After upgrading we noticed the max file descriptors was capped to 4096 in the log which is not inline with the config file or the settings on the server. It does not look like the config setting is being acknowledged which might be an issue with accounts with lots of small docs.
2017-07-10T09:49:13.855Z ==== Couchbase Sync Gateway/1.4.1(3;ddffd85) ====
2017-07-10T09:49:13.855Z requestedSoftFDLimit >= currentHardFdLimit (250000 >= 4096) capping at 4096
cat /proc/sys/fs/file-max shows
As the sync gateway user max file descriptors is correct as per your doco.
sync_gateway@sg02-ln-uk-p:/var/log/sync_gateway$ ulimit -n