Some spin job on my part when I say that Granderson had a much higher OPS+ than Matsui in 2007 and 2008.
135 to 123
123 to 108
Matsui also missed a lot of games in 2006 and 2008. Granderson is a lot younger, has been durable, and isn't limited to being a DH like Matsui. It could be a downgrade. It could be a wash. Or it could be an upgrade. The point is it's not really definitive.