What are the pros and cons of publishing a positioning paper based on a grant proposal?
Publishing a position paper can possibly be useful to get feedback about the idea and refine it for grant submission, particularly if you're publishing it at a conference.

I personally don't worry too much about getting scooped either, but I would likely only do it if it lays out a space that is far bigger than can be tackled in a single grant proposal. Sure, someone else might get inspired - I think that's a good outcome! - but it isn't terribly likely that they're going to go work on the same corner of the probelm that you are.