I think one thing you have already identified, is that certain lenses aren't for you. If you believe you will be doing most of your shooting in the 80-400mm focal length, and you want a one lens solution, and you're not worried about large constant apertures, then the new 80-400 would probably serve your needs very well. I've heard very good things about the new version and if I had the spare money, I'd probably pick one up myself.
I think something we are all guilty of doing from time to time is over analyzing every possible lens combo for one advantage over another. Bottom line, to me, is if this focal length is best suited for you, and you have the money, go for it. Though I would recommend renting one for a few days before buying it, just to make sure. It might cost you $100 or so, but it would be worth the money to know for sure if you want it before buying it. Think of it this way, spending $100 to rent it is much less than you will lose on depreciation if you try to sell it right away because you don't like it.
Just my thoughts, hope it helps.