We held off on Qt 5.8 because of problems with the Wayland support between it and KWin. Qt 5.9 seems a little more promising but we don’t want to change something so fundamental to KDE’s software without checking for issues so it’s still in the hidden testing repo for now. Try it out on KDE neon Developer Unstable edition to see what breaks.
deb http://archive.neon.kde.org/testing xenial main
If you’re brave check it out for problems. (The packages even seen to work on the other editions of KDE neon but you are on your own there because it upgrades KWin and other parts to non-stable builds.)
Please do test out a Wayland session with it too (log out and select from bottom left of screen). And review the Plasma Wayland bugs list.
Let us know how you get on using this blog, or neon mailing list or bugs.kde.org or forums.kde.org as appropriate.