FWIW, and at the risk of adding to the profusion of apps in everyone's toolbag, I do tend to sketch and refine a project as an external outline before committing to OmniFocus, mainly because (while I find OF quite wonderfully solid and dependable), I do also find it a little bit slow (certainly as an outliner anyway - part of its solidity is that a lot of prudent house-keeping seems to go on in the background each time we indent or outdent).

I use a plain text outliner for developing projects (FoldingText), and I have drafted scripts which:

1. Export from FoldingText to OmniFocus
2. Import OPML into FoldingText
3. Export OPML to FoldingText

(No guarantees – roughly hewn and intended for my own use – but available anyway on Github).