Based on the current information they will feature the same stock base speed and slightly lower boost speed capping at 4.2Ghz. If they really follow the 10% enhancement trend this results in identical performance with the only stock improvements of supporting DDR4 RAM.
On the flip side they can handle the same 95watts of current gen i7 CPU's which will mean they likely overclock better due to lower power requirements. This is tricky though, we have no idea how it can handle higher core speeds with their memory controller.
I'm running the 4770. So I will be waiting until the Cannonlake before I make the change.