I’ve been using a Sager (aka Clevo) i7 with dual GTX870Ms in SLI (see below) for a while now, and it kicks serious butt. “Portable” is a relative term though, as this thing is heavy, the power brick is about the size and weight of a regular PC power supply, and battery life is about 50 minutes when the system is loaded.
I have had mixed results with the Rift depending on which SDK version; some of them have worked perfectly, some with micro-stutter (0.6.0.1 is the sweet spot for the DK2 and vvvv). I have not tried the released version yet. All the Unity and Unreal demos run flawlessly on it.
Unbelievably there are notebooks out with a REAL GTX980 in them, but they ain’t cheap.
A note about SLI: at least on the systems I’ve seen, that doesn’t really buy you much as that may only work on the main laptop display, not an attached display like a Rift. You may be able to set the main display to the outboard display though and get it to work that way, but it’s a hassle.