Given that the PB + XFR can hit 4.3 GHz, and some users have reportedly actually gotten 4.4 GHz or even 4.5 GHz out of their 2700X chips, I'd say it's safe to bet that AMD could drop a 4.5-4.6 GHz 8 core pretty easily. Intel at 8 cores is either going to need extreme TDP to be able to stay at 5 GHz or it will likely have a slower boost than the 6700k...a Mythical 2800X vs this supposed 8 core chip would be a much closer race than you'd believe. Intel leads in clockspeed, but it also has higher TDPs than AMD chips. That becomes exponentially problematic as you increase cores.
By comparison, most users needed 1.375+V to hit 4.0 GHz depending on the chip. Meanwhile, the 1950X binned cores could do the same clock speed at 1.2V in many instances...and that's with 16 cores! My 1950X is capable of pushing 4.1 at 1.325-1.35V (can't remember, have an odd issue with my AIO that causes the CPU to heat up during prime95, but it worked fine on my other board). I believe 4.2 GHz was 1.45V - 1.475V, however, the motherboard could not handle pumping that much power and throttled. Just based off current data, you can expect that AMD could compete pretty easily if they want to. My thought? It depends on Intel's release of the 8 core chip. If it's close to time to clear inventory, I fully expect them to just clear inventory and wait for zen 2. If Intel releases it in 1-2 months, we'll probably see a 2800X.