Nope. The exposure would be different, but there are tons of other factors besides what the lens is made of. For example, most lenses are made of glass, but not all glass is alike. The same with plastic. There are coatings for UV, IR, color correction, indices of refraction, focal lengths, aperture blades, et al.
In your example with the camera with the plastic lens, the only real way to tell which lens is faster is to do a proper test. The plastic lens may be made of the right material to transmit light and have no other inhibitors and could actually be a faster lens - albeit with a limited application range. If its just the one element, its likely the modern equivalent of a pinhole camera and the plastic lens is just a light gathering device.
Don't judge a lens by its case. Remember: the mark of a good photographer is not in what they can do when they have all of the perfect equipment, but when they have none of it. If you want some real fun, go pick up a Holga - medium format film camera for only a couple bucks. Plastic, light leaks, super cheap.
Jonathan Ziegler
http://www.electrictiger.com/
520-360-8293
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