Perhaps Sigma will be out with their new 120-300mm f/2.8 OS Sport lens before long and throw in a 2x converter with it.
I personally got tired of waiting and got the 300mm f/2.8L IS II: cheaper and works a charm with both v3 teleconverters.
Cams: Canon 1Dx, 6D, EOS M
Zooms: 17-40mm f/4L, 24-70mm f/2.8L II, 70-200mm f/2.8L II; Primes: Sigma 35mm f/1.4, 40mm f/2.8 STM, 50mm f/1.2L, 85mm f/1.2L II, 100mm f/2.8L, 135mm f/2L, 300mm f/2.8L IS II, 600mm f/4L II
Support: Gitzo GT4542LS/GT2830/G2258, RRS BH-55, Wimberley WH-200