It seems mythtv can be built against QT 5.5 and QT 5.6 but the ebuild only allows building against QT 5.5 due to it's DEPEND lines. This was a problem for me because the latest KDE 5.x versions want QT 5.6 but the mythtv ebuild blocked that. I updated the mythtv ebuild to require QT 5.6 which resolved the block. To prevent doing an upgrade to QT 5.6 if someone doesn't want it though I think the ebuild should allow for either QT 5.5 or QT 5.6 in it's dependencies.
Right now mythtv-0.28 has no Qt dependency restriction. What is does have is the subslot operator which in certain cases due to a bug in portage can make it appear as if it's pulling in an old Qt version. If the issue persists, please reopen this bug and add the full portage output.
