The data on notebookcheck shows the 555 being slower than the 455. However, the data there isn't necessarily gathered consistently (equivalent CPU, same drivers, same settings, etc), so I doubt it's an apples to apples comparison. AMD's official specs show the 555 and 560 to be virtually identical to the 455 and 460, so I'm very doubtful of any reports showing substantial gains or losses between the generations.
Looking at notebookcheck, the 460 usually beats the 455 by about 10-20% for game frame rates. But again, I don't know how rigorous that would be.