There is a difference between rationality (optimal decision making based on the known facts and a set of value-weightings of possible futures) and the value-weightings themselves. It strikes me that eating the peach would quite probably alter how much you would value particular possible worlds, but I find it much more difficult to believe that it would alter your ability to deal with complexity (predict the future).
Here are better primary sources:
The Johns Hopkins Study: http://www.hopkinsmedicine.org/Press_releases/2006/GriffithsPsilocybin.pdf
"How can I be more rational from a drug?"
In the short term, ingesting sugar will boost your capacity for handling complexity.
"I doubt a psychedelic experience can help me optimize my current utility function better than my sober self. How can I be more rational from a drug?"
WTF, dude? Not everything in life is about improving your rationality. Do you expect to become more rational after eating a hot dog? How about a peach?
The entire point of being rational is to maximize your expected life value.
If taking psilocybin increases your expected life value by construction you would be irrational not to take it... unless you place a high value on passing up "at least a moderate increase in well-being or life satisfaction".
David Stove also talked about it a bit (not focusing on the transmission part but more on detection) with "What is Wrong with out Throughts?" ( http://web.maths.unsw.edu.au/~jim/wrongthoughts.html ). I'm not sure there is a good solution, as it is almost impossible to know whether or not you are in the grip of some irrationality.
To give an example, it is conceptually easy to kill germs - bacteria simply can't handle wide swings in humidity, temperature, acidity, etc. Washing hands with soap and hot water, cooking food, using bleach, etc. are easy ways that are guaranteed to kill bacteria. They have an extremely low failure rate (Anthrax is the toughest bacteria I know of, and it can be killed with enough bleach and ingenuity).
These limitations are caused by limitations in the fundamental processes that make life work. Metabolism has to happen in particular temperature ranges. Cell walls can only be made out of a few sorts of materials, and all of those materials react violently to extremely basic or acidic substances.
The basic problem is that, if there are analogous limitations to "mind viruses", we simply don't know what they are (beyond the trivial making the host commit suicide instantly).
The best I have come up with is the advice that Feynman gave in his "Cargo Cult Science" talk ( http://wwwcdf.pd.infn.it/~loreti/science.html ) - cultivate a brutal sense of honesty so that you have a small edge on the detection side of things.