Provided IQ is close to excellent, for an amateur phtographer with wild-life ambitions what would be the alternative to spending more than 2k for a (highly speculative) mark II version of the Canon 100-400? Sigma 120-300 f/2,8 is too short in reach and @ f/5,6 not superior to the current Canon 100-400. The other serious alternatives are way too expensive. Unfortunately 
As someone who has been looking to get into this area and that has been eyeing off the sigma120-300/2.8 OS vs 100-400, I'm curious as to your comparison?
Outside of weight the sigma has looked better to me given the potential extra reach (420mm with 1.4X or 600mm with 2X TC)
Are you saying that sigma @ 300mm + 1.4 TC (effective 420mm, F4) is optically worse than the 100-400 @ 400mm? Even when the sigma is stopped to F5.6 equivalent?
Or is the comparison based on the sigma with a 2X TC? In which case 600mm = substantial reach advantage over 400mm...