I want to focus on the goal of producing "top end images". Except for in very narrow, highly-competitive special cases, you certainly don't need a top model camera to do so. (There's an old adage — cameras don't make photographs; photographers do). And, since you're talking about going back several years in technology, there are many areas where your current camera outperforms or is otherwise technically better than the older higher-line camera (see Matt Grum's answer).
You don't say what other equipment you have — maybe you are blessed with a collection of L glass and have a fully-outfitted portrait studio (or portable lighting gear to set one up anywhere). If you don't have great lenses for the photography you want to do, or if you haven't explored what you can do with higher-end flash photography, those things are very likely to be what you really need to do to for more top-end images. The 5D Mk II is a workhorse camera, and I'd recommend using it until it really does fall apart, saving a little for the replacement but putting the bulk of your budget into these other things.
So, fundamentally, I think it is unlikely to be worth it. When the time comes, a newer 5D Mk III (or IV, by that point?) is probably a sensible choice, but you might actually even consider stepping down a notch to the 6D line, if you can use the money to build up the rest of your gear instead.