REAL unpopular opinion time: there is no caster/non-caster gap. It isn't a problem.
Its there its just a highly variable table problem. Its an emergent problem that hinges on what parts of the game are being utilized and to what degree.
And even then, theres a clear underlying culture problem thats exacerbating it as well; ie, most people basically admitting their caster players are gigantic egomaniacs who hog the spotlight even when they don't have to.