Rather than wait I got myself a 2nd-hand 100-400 for Xmas and went a'shooting in Kenya. The idea is to wait for future announcements, reviews & maybe price drops, and then sell the 100-400 to offset whatever comes next. A couple hundred euros or so for a years rental seems worth it. Really happy with the results, though my only other experience with L lenses is my trusty 24-105. Before that I was a Rebel user. Would have been cool to have a 1.4x TC but had already spent too much. Pull zoom didn't completely lock, this looks like a common issue as most 2nd-hand lenses on the site mentioned this defect. Not too big an issue in most cases, this isn't exactly a walkaround lens.
Hand held leaning on the van roof (roof comes up) didn't even use the beanbag I brought along. Glad I read ahead and learned the tripod would have been pretty useless. Main difficulty was handling my kids at the same time ;-)