I looked up Spielberg's co-writers, and they don't seem to have written anything else so significant (closest might be Cool World). I think the traditional auteur theory often neglected the importance of the writer(s) involved.

Still, if you look at Salem's Lot and The Funhouse (pre-Poltergeist) and Lifeforce (post-Poltergeist), none of which are written by Hooper (which seems to be the case on other films as well), the director most famous for Texas Chainsaw Massacre is quite adept at achieving what the screenwriters brought to the table. My guess is that the degree of contribution is close to 50-50, with a slight edge toward Spielberg, who has established many of the Poltergeist themes in his previous and later films.