Yes, they will perform better in terms of raw power. They will also require a larger power supply, as well as produce a lot more heat. You'd also potentially be stunted by lack of VRAM depending on which versions of these cards we're referring to.
If you're going to crossifrex some ATI cards, I'd suggest at least R9-280 non X's for their price point and extra video memory for larger resolutions. Also carries more viable weight into the next generation of games. Assuming you weren't just asking this for the knowledge of it alone. I'd go with a single GTX 970 over either of the aforementioned solutions