Results: DiRT Showdown
A persistent platform bottleneck keeps the move from two to three Radeon HD 7970s flat, devoid of performance gain at 1920x1080.
With two cards installed on our X79-based motherboard, the minimum frame rate takes a hit. But the average is even with the other two configurations.
It's common sense, but if you're shopping for more than two high-end graphics cards, make sure you're gaming at 2560x1600 or 5760x1080. These numbers show us that scaling resumes once you start using a demanding-enough resolution.
There's really very little practical difference between our three tested configurations, suggesting that a single 16-lane slot going out to three Radeon HD 7970s is ample.