Conclusion
Assuming you're swimming in cash and lucky enough to catch the Titan X in stock, $2400 gets you the best gaming performance that money can buy.
The card’s scaling in two-way SLI is pretty good; expect 70-80% over a single card (depending on the game) at 5K. If you plan on playing at 1440p (or lower), you’re more likely to see a 40-50% speed-up before your CPU becomes the bottleneck at higher frame rates.
We've also proven that you want that $40 SLI HB bridge on top (something that GTX 1080/1070 owners will appreciate knowing), else you may face a 25 to 35% performance hit in certain games.
For VR, you're better off with just one card at the moment. It'll be a while before developers start taking advantage of multiple GPUs—growth on that front has been slow thus far. Still, a single Titan X is capable of running Elite Dangerous using the Ultra preset at a steady 90 FPS on an Oculus Rift (although motion-to-photon latency is still a sub-optimal 27 ms.)
Whether your rig is a $500 value box with second-hand-parts scavenged off of eBay or a $10,000 behemoth, we'll leave you by the same letters Nvidia inscribed inside the box of the Titan X:
MORE: Best Deals
MORE: Hot Bargains @PurchDeals