I have had great results in some studios with carpet over concrete.. all depends on the rest of the studios construction / accoustics IMHO.
In fact the studio owner at one place pulled up the carpet, put in some (fake wood) flooring to make a nice 'looking' drum area - and it messed it up totally..
You have to creep up on this sh!t in case you wreck everything that was good about it and make no additional improvements