Are you playing SE or Oldrim? If you are playing Oldrim, I don't understand how you are getting past the 4GB VRAM limit. Maybe your VRAM monitor is detecting VRAM usage for applications in addition to Oldrim?
There's been some noise on Reddit about the possibility of Microsoft patching the DX 9 limitation out of Win 8 and Win 10 but, as someone else put it, it might just be vaporware.
Finally! I will try it today, hope it really works with the new preview build.