the Doc is for quickly jotting down ideas and short texts and nurtering them into a states that makes them deserve to inhabit an own file. I would like to get rid of the Word-Doc, but you can’t really write longer texts in OmniOutliner.

I really recommend VoodooPad for this. Documents are spotlight-indexable and you can sketch right into the pad alongside your notes, drag images into the pad, files into the pad so they are invoked when clicked. Export a number of different ways. In fact Gus Mueller's weblog is generated by VoodooPad.

DevonThink is a Data Mining organizer, I don't look at it as a place to organize your thoughts. You could try Tinderbox, but the learning curve is very steep, the interface is antiquated, and I'm not sure how much help it is when contrasted against the tme spent massaging the agents.

Brainstorming to me is about taking snippets of thoughts and figuring out how they fit together -- outliners don't do that for me, they simply break down a bigger idea into smaller chunks (also necessary, but I look at this as more useful once an idea is already clearly defined). Data correlation is best visualized in a form that already naturally supports self-documentation. As you flesh it out, it grows together... a wiki is the best fit.

Braindumping in a kGTD document seems like a great way for information to get lost in the shuffle. Probably better to keep your notes in something else and drag them next to your action item. Spotlight doesn't become as useful even though OO documents have a mdimporter plugin when ALL of your projects are maintained in a single OO document.