As much as I want this rumor to be true (and an announcement following soon, so that I can stop eyeing the Sigma 150-600 S ;-)), I don't see all three of the "f5.6", "600mm" and the "cheaper than 100-400II" become true at once. If this rumor has any foundation, it will most likely be a direct equivalent of Nikons 200-500 f5.6.
In any case, this rumor makes the dilemma I am in even worse. Last year, I tried both the Sigma 150-600 C and the Canon 100-400II on my 7D(I). Neither one convinced me, the Sigma was good on the short end, but the long end was severely lacking in crispness. The Canon hat severe CA on the left side of the frame and IQ didn't "wow" me overall to justify the price (maybe mine was a dud?).
In any case, currently I am pondering giving the Siggy 150-600S a try, but then again, it is so close in price to the 100-400II (~1550€ vs. 1900€) that I also consider giving the latter a second chance.
Further, when looking at the the-digital-picture comparison of the Siggy S, it looks outright horrible compared to the C throughout most of the focal range (maybe
his was a dud?) and it weighs a ton, definitely not something I can just drop into my carry-on luggage and have room for anything else or would lug around all day without at least a monpod as support.
Spoiled by choice :
And now there is this rumor... maybe I just buy anyone of the above mentioned lenses, and once my return period is up, Canon will announce the best long telephoto zoom ever. ;D