1. RHEL can't deliver KDE because a) it employs almost all GNOME developers and b) KDE needs to be updated to work reliably, and RHEL can't deliver updates as fast as needed. RHEL 6 uses KDE 4.3. That says all.
2. Debian had issues packaging KDE. If you want KDE 4.11, you want Debian Testing. SteamOS uses Debian Stable.
If you want an enterprise level distro featuring KDE, you want SLES/SLED. SLES/SLED 11 featured KDE 3.5.10 with an extensive SuSE patchset that became the basis of the only true bugfixes delivered with Trinity. SLES/SLED 12 is going to be the release with an enterprise-level Plasma Desktop, so, that's going to be the one to watch.