AMD will need to keep something up their sleeve for a potential Intel fightback, so if they managed to get a 10-15% IPC uplift, I doubt they'd also increase clocks by that much and up the core count. Upping CCXs to 6-cores and therefore being able to release 12-core mainstream chips, plus a 10-15% IPC increase, would mean they can compete easily without any clock uplift at all. In fact, they could keep lower clocks on all but their -X series chips and boast better power efficiency. That doesn't mean the chips won't be able to overclock higher though, that might happen regardless.
8-core CCXs are also interesting since it means they can release an R7 3700X equivalent with just a single CCX, eliminating an infinity fabric issues that have affected performance in certain applications (games mainly). I still don't think they'd release a 16 core mainstream chip if they managed this though. Absolutely no reason to cannibalise their HEDT platform since Intel has nothing anywhere near this right now.
However, all rumour and "don't believe it until I see it" is my default response.
