If are willing to file a bug report https://github.com/RasPlex/RasPlex/wiki/How-to-report-a-bug then i'm pretty confident that we can get you up to snuff. 1080p is working flawlessly for most people, probably has something to do with your network setup.
Devs - have you seen the post regarding the new PHT 1.0.8.
Seems like they have fixed a number of issues that people are having with 9.9.16, 9.9.17 and 9.9.18.
Regards
They have, see 0.9.9.18 release announcement on Facebook.
"RasPlex *experimental* build 9.9.18 is ready.
It’s a fairly minor update, but it fixes another major network flooding issue seen by some users, and includes PlexHT 1.0.8 which is hot off the presses."
After an hour of testing it seems to handle 720p DTS high bitrate via HDMI passthrough, handles 1080p DTS on same.
Set up using MyPlex PIN, waited for pre-caching to finish, set my HDMI passthrough, timezone and format. Browsing seems pretty snappy, better than before I think.
I'm running 9.9.18, fixed iOS remote but still having network flooding issues using wireless. Grinds my whole network to a halt. Wired connection is fine.
I just wanted to report that with 9.9.18 the remote was indeed functional again. However, I was getting choppy playback. I reverted to 0.3.1 and playback is fine again.