With the same IQ as the 70-300L and IS, I would by it, even if it is bigger and has more weight than the 70-300L
Love to have this zoom reach on FF (hope it is EF)
If they make it EF-S it would be a very strong statement for future crop sensor support! I doubt it though.
5DIII, 1DX, 8-15/4L, 24-70/2.8L II, 70-200/2.8L II, 70-300/4-5.6L, 200-400/4L 1.4x, Zeiss 15/2.8, 17/4L TS-E, 24/3.5L TS-E II, Sigma 35/1.4, Zeiss Otus 55/1.4, 85/1.2L II, 135/2L, 600/4L II