further update, early last fall I added the 100-400 Mk II (nice!) and the 11-24 (another story). wow. I still use the 35-350 - but mostly on a monopod - specifically, I've used it at botanical gardens.
the 100-400 II is amazing. it's a game changer. if I need wider than 100mm (FF), I'll use a second body & either a prime (35mm IS, 40 stm or 50L) or one of several zooms (8-15, 11-24, 16-35/4, 17-40 or 24-105). my sherpa/cfo/better-half prefers an X100 or an SL1 w/pancake).
one day last summer, I mounted an evaluation copy of the 100-400 II on her SL1. talk about odd. that spun a head or three. but it really worked well. productivity wise, it was a good day (I also had an X100s). the little SL1 never got in the way. after the first half hour (developing familiarity with the combo, upgrading muscle memory, etc.) it was a decent setup. kinda surprised me.
meanwhile, as rumors swirl of an upcoming replacement, my interest in the 28-300 IS has faded. I can't forget getting beat up that day. phew, it was not a fun shoot.
also, for more casual outings, I've added the Tamron series: 28-300 di i vc pzd (FF), 16-300 di ii vc pzd (crop) and 18-200 di iii vc pzd (ef-m, for the m3). serviceable optics for what I describe as "companion" lenses, just not L quality. I still like the older tamron 70-300 vc on my 1d3 - but a real tank compared to an M3 w/tamy 18-200 (see happier sherpa).
my main point is that the 100-400mm mk II is in a league of its own. while somedays I still reach for my 200/2.8 mk ii - and I still think about a 200/1.8 or 2 &/or a 300/2.8, etc. - but this new 100-400mm is really, really nice.
I'm no longer sure of my need for the 28-300 or its (when the spirit moves them) upcoming replacement.
enjoy & ymmv...