According to SiFive, the P870 is roughly equivalent to an ARM X2 or AMD Xen 4, so it's about two or three generations back from the Cortex X925.

I've yet to see a catalog part w/ that core, but the guys I know at SiFive say they're pretty sure it should hit 3GHz, which is cortex X2 territory. Not exactly ARM's current best, but they're definitely closing the gap.

One difficulty in evaluating SiFive's cores is their business model is to license their designs to companies who are fabbing their own custom or semi-custom SoCs, so they don't appear in generally available parts until much later.