specific numbers aside, I agree with your sentiment. Right now the big 4 of today are just as good as the big 3 of 2008 + a very inconsistent Rajon Rondo.
Rondo's game improvement over 2 years is huge.
Ray is about the same, if not better - remember he was playing injured in 2008. His peak as a Celtic was in 2009.
KG and Pierce have dropped off a little, yet are still about 5th best in the NBA at their positions.