I'm sure there are "proper" studies into this but i certainly know a lot of people that think people being "PC" is just a cover up for people. It's a learned behaviour about what it's acceptable to say but doesn't necessairyl help to change people's thinking. Black friends I have say they preferred it better before people were PC that way they had advanced warning as to who the racists were, now you have to scratch a little deeper to find outI agree with you to an extent--often, I found that people often invoke PC as the last line of argument, when they don't know to adequately respond to your point.
But is PC that pervasive in everyday media, though? The example of scientists afraid to publish certain types of studies about gender and race--those still do get published and discussed. For example, I remember reading a news article recently discussing a study in which two scientists concluded that Jews were the most intelligent people. However, studies like these do get criticized, not necessarily on the grounds that they are un-PC, but because they are often scientifically unsound.
I think a tempered form of PC has some use in everyday life--after all, it can shut the real bigots and racists up. Extreme PC, though (as Ray Bradbury would argue) should definitely be frowned upon--but then again, extremism in the majority of things in life is not exactly desirable.