Need benchmarks, but I think it would be safe to assume 5-10% faster than the A8 on CPU intensive tasks if just because a (likely) higher clock speed. GPU is harder to judge, but I'd think somewhere in the neighborhood of 10-20%.
My guess is that the A7 wasn't quite enough to handle the Retina Display in the A7, although it wasn't bad, certain animations, such as launching Siri did lag a little. I think that's why it's an A8X. With 250% faster GPU over A7, it's crazy fast.
Need benchmarks, but I think it would be safe to assume 5-10% faster than the A8 on CPU intensive tasks if just because a (likely) higher clock speed. GPU is harder to judge, but I'd think somewhere in the neighborhood of 10-20%.