I'd say to answer that worry, most, if not all of what has happened so far is pretty believable, at least in my eyes. Obviously right now genetic manipulation is in its infancy, but the idea of improving certain features such as flexibility, bolstering hormone levels and the like isn't an idea that can't be contemplated. Heck, even flavoured milk isn't a long stretch- I could imagine manipulation of the genes to direct the body to synthesise specific glycoproteins and carbohydrates, with the right additional nutritional intake, that would flavour the milk accordingly (though mocha milk<em> maaayyy</em>be pushing it a bit, however delicious!). The most far out idea has been the tail and ears, but that little tidbit can be ignored (but, as you say, too many must be avoided).