This was said by someone on the G3D forum. I couldn't have said this better, so I am quoting:
"I find the 15% claim too good to be true. According to Guru3D's benchmarks, the 9900K is only about 3.4% better in IPC compared to the 2700X. A 15% boost would mean Zen 2 will have a 11.6% IPC advantage over the 9900K!
A 15% boost in single-threaded performance would be more realistic (that would make it nearly as fast as the 9900K), based on both IPC and clock speed improvements."
If this turns out to be true we are going to see an interesting price war, hopefully.
i7 7700kK @4.2ghz
16GB DDR4 3200mhz
GeForce 1080 Ti