Hey there! Just discovered the project poster and I think its awesome. Especially the problem space is definetely the thing we were missing a little bit whie we dove too fast into the solution space 
But we have (yet) a small product and I wondered if you guys would create a project poster for each new feature? Then it would be more a feature poster. Does that make sense? 
Also, du you then create a product requirements page (solution space) as a child page within the project poster?
Cheers, Lars