Yes, the Canon 1.4x fits - at the same time as I hired the Sigma, I also hired Canon 1.4MkII. I wasn't entirely convinced, but then we have been getting equinoxial winds (read gales) and the lens hood ("bucket") collects an awful lot of it! It just wasn't as sharp as some shots I've seen using Sigma 1.4x, but then I don't know how much pp they've done.
Thanks for sharing your experiences
5D3, 5D2, Sony α6000, G16 | EF: SY14/2.8, V20/3.5, 28/2.8 IS, Ʃ35/1.4, 50/1.8, Ʃ50/1.4 EX, 100/2.8L IS Macro, 16-35/4L IS, 24-105/4L IS, 70-200/2.8L IS II, 1.4x II, 2.0x III, 70-300L IS, T28-300 Di VC PZD, Ʃ150-600 DG OS HSM S | E: SY12/2, 35/1.8 OSS, 16-70 ZA OSS, 55-210 OSS, SY300/6.3 ED UMC CS