Yeah, I agree. Obviously the focus of this jam should be SFML, but I think the priority is still to make great games.
Let's try this instead.
Just like before, rule 5 get's changed to: "External libraries are allowed, but not SFML's competitors, such as GLFW, SDL, Allegro, etc."
And the new rule becomes: "Everything graphical that SFML is capable of must use SFML. OpenGL is allowed for more complicated graphics, such as 3D, and for performance reasons."
I'm still not quite satisfied with the wording, but I like it better. Also, I think we can let "performance reasons" be left up to the participants. Like you said, using OpenGL is a gamble. They can decide if it makes sense to use it to get that performance boost or not.