Two years ago I experimented with a very good quality mylar (could have been 11 mils). Wanted to see if it would reflect enough light on the underside of the foliage to deter aphids. Very disappointed results. In less than 2 weeks the mylar lost it's shine and became permanently dull as a result of being exposed to the elements.
I am more interested in being able to reuse the sheets than I am about the reflectiveness of the Mylar. I doubt if there is enough light is reflected from the small surface area of the shower cap to make a difference. I think you would have to lay out a sheet around the plant to get that type of effect and most likely it would only last a short time.