In a forum thread dedicated to discussing the Skylake batch and steppings, Slamms from Russia shared his initial findings of testing 18 engineering samples. Amongst the 18 we find only one Core i7 6700K. The list of processors contain the following Skylake steppings: L510C971 (16), L512C763 (1), and L514B860 (1). The test consisted of finding the lowest possible voltage to boot at 5GHz and checking if it can boot to the Windows 7 operating system.
From the 18 tested CPUs five made it to the operating system at 5GHz. On average it takes about 1.5V on air cooling to accomplish this, which is quite a bit higher than what we saw with Haswell. A typically great Haswell would achieve 5GHz to OS at 1.2V or lower. The VID of the processors varies between 1.168V to 1.26V.
For more information and the full details on the eighteen tested CPUs, check out the thread in the Skylake OC sub-forum.