The performance gap is a bit larger than that, depending on motherboard used.
A 6700K on a Z-series board will run all 4 cores at 4.2GHZ due to "Multi-core Enhancement".
The Xeon will run 1 core at 4.0GHZ at best. All 4 cores will probably be 3.8ghz. This is, of course, assuming Intel didn't change how Turbo operates on Skylake/Greenlow.
My Xeon E3 1230 V3 is 3.3 base / 3.7 boost, but only operates at 3.5 boost on all 4 cores in most applications. On certain punishing applications, it actually drops straight down to the base on all 4. This is on a C226 board.