As of today, I am convinced that the Surface Book will in all likelihood never support dual 4K displays at 60 Hz. Today, the SurfaceSupport Twitter account linked me to a support document that clearly says that the device is limited to 30 Hz when using two 4K displays. I'm not sure how long that document has existed. For my own peace of mind, I hope it's relatively new. 
Up until this, I had been hopeful that the limitation was something Microsoft could and would work around.
It's probably a hardware limitation, as others here and elsewhere have said. But it took seeing it clearly stated on a page on Microsoft's own domain before I accepted it.
I will probably keep the Surface Book, but I'll buy a "proper" workstation to power my monitors and use the Book as a mobile-only device. I do quite enjoy it. I simply wish it had been what I hoped it to be.
I ranted a bit more in a blog entry: Microsoft concedes Surface Book incapable of dual 60Hz 4K / Tiamat
Up until this, I had been hopeful that the limitation was something Microsoft could and would work around.
It's probably a hardware limitation, as others here and elsewhere have said. But it took seeing it clearly stated on a page on Microsoft's own domain before I accepted it.
I will probably keep the Surface Book, but I'll buy a "proper" workstation to power my monitors and use the Book as a mobile-only device. I do quite enjoy it. I simply wish it had been what I hoped it to be.
I ranted a bit more in a blog entry: Microsoft concedes Surface Book incapable of dual 60Hz 4K / Tiamat
