I just finished reading Yes! 50 Scientifically Proven Ways to Be Persuasive (hat tip to the [non]billable hour). One of the 50 ways confirms something Bryan Garner says in The Winning Brief (p. 177, 2d ed.): people who use plain language are perceived as smarter and more persuasive than people who do the opposite:
“... Take, for example, the fact that communicators frequently try to convey their erudition via their grandiloquent, magniloquent, sesquipedalian verbosity; in other words, they try to look smart by using unnecessarily long words or overly technical jargon....
“... [R]esearch by [Daniel] Oppenheimer has shown that using overly complex language like this can produce the exact opposite of the intended effect: Because the audience has difficulty interpreting the language, the message is deemed less convincing and the author is perceived to be less intelligent.”
In an endnote, the Yes! authors cite D.M. Oppenheimer, Consequences of erudite vernacular utilized irrespective of necessity: Problems with using long words needlessly, Applied Cognitive Psychology, 20:139–56.
So there you have it: scientific proof that plain language aids persuasion.