The other serious alternatives are way too expensive. UnfortunatelyWell, the 400mm f/5.6 is well regarded, but, obviously doesnt have IS. Maybe the 300 f/4L with a teleconverter? Or you could stick with the original 100-400. All are around the same price and have sufficed for many a wildlife shooter.
I'd be interested in your thoughts accounting for (presumed) the weight increase..A couple of guesses, based on the rumors and patents. For one, it'd be 19 elements in 14 groups instead of the 17/14 of the previous lens, so extra glass means more weight. The different body style and weather sealing could add weight. And if it truly is 82mm instead of 77mm, then the glass itself is larger, which would be more weight. It doesn't say how much more weight...so, even like 3.25lbs would be considered "heavier", even if its truly not "heavy"