I was considering snagging a 3900X at $399, but I may just wait for the 4900X or whatever chip has an oversized L3$ per core for my DC workload if the IPC really does hit a 15% boost and the power draw is essentially the same. An IPC gain of 15% tells me they're probably not going to N7P, but rather N7+, even though it seems like they're trying to backtrack on their slides.
No, it will be N7P. AMD has already stated this and they have also shown shown that there is plenty of performance to be had from optimizations. Look at Vega vs Renoir and Navi vs next gen. I strongly suspect that this next gen is all about power/performance optimizations, clock speeds, and feature enhancements.