GPU and Gaming Performance
For gaming at 4K, you're going to need every last drop of the GTX 980M's performance. The flagship GTX 980 desktop card struggles in some games at this resolution, so for mobile gaming at 4K and decent quality settings, you won't want anything less than the most powerful mobile GPU available.
The game I played most on the Alienware 15 was Assassin's Creed IV: Black Flag, as I hadn't finished it and it's well optimized for 4K gaming. At a resolution of 3840 x 2160 I ran the game on mostly high settings (one notch below maximum), and achieved frame rates around 40 FPS, which was adequate for this title. With such a high resolution display, the game looks excellent and is highly detailed, so even though you're not running the game at maximum settings, you're still presented with a fantastic gaming experience.
If you'd prefer better frame rates but still want to game on maximum quality settings, gaming at 1080p is an option as it scales the nicest of any non-native resolution. I tend to prefer to always run my games at native resolution as it looks the best, and at 4K you don't have to crank up the anti-aliasing, though it's always nice to have the option.
Being able to game at decent quality settings and at 4K on a laptop is an impressive feat considering how powerful the hardware needs to be. Many gaming desktops out there that TechSpot readers have built won't be able to handle this resolution sufficiently, so to have this power on hand in a portable system (albeit an expensive one) is excellent.
To tell the full story of the Alienware 15's performance, let's look at benchmarks. Note that for some game titles I benchmarked at 1080p (the default for out tests) as well as at the native resolution of the laptops, giving you an idea of what performance hit there is from the increased display resolution of the Alienware 15.
As you can see, the GTX 980M is between 2.0 and 2.5x more powerful than the Maxwell-based GTX 860M in the Alienware 13, making the overall top-end Alienware 15 system (including CPU gains) around twice as fast. Gains aren't as large compared to the last-generation GTX 870M, but still significant considering this is a mobile platform with similar power constraints.
When it comes to gaming at 4K, the Alienware 15 struggles to run our test suite of games at maximum settings, which is understandable considering the aforementioned requirements to game at 4K. However if you reduce the quality a little, gaming at Ultra HD is definitely achievable when paired with the GTX 980M.