I'm not sure whether you mean "off topic" in the sense of other related technologies (C++, OpenGL, ...) or completely unrelated discussions (real life, politics, religion, ...) but especially for the second I don't see any need.
With the exception of a few people, the current model works relatively well, but I agree that one could be more strict with respect to pure C++ questions, as there are still many beginners who barely know how to write a function and flood the forum with corresponding questions. "More strict" might mean refer to a good tutorial/book and not answer the actual question, as that only encourages having no self-initiative. Of course, it's difficult to find the border, but there are enough cases where it's obvious that a poster just doesn't feel like reading a C++ book and wants the community to work for it. Some people
even admit it.
In general, the community has been very tolerant in the past, even if a topic is only slightly related to SFML. For example, there have been a lot of interesting discussions about game development and various related techniques (components, collision, pathfinding, steering, AI...), and I think it's good to keep that. I don't know whether an additional forum would change a lot here.
