I didn't find an actual answer, more a work around. As this was all part of a bigger, involving plex, I found a proxy (xteve) that includes ffmpeg transcoding, and I can set the options for ffmpeg. That is good enough for what I was trying to do.
Hope this save someone else a little time.
Wireless will always give you some latency (if you're not spending a fortune). I would recommend you try using sdi cables. In video-production, sdi is often used for immediate feedback to the directors monitor etc. - so you should have less of a problem with latency there.