The compression is different. The Cad Version has 10:1 ratio and the other ones have 8.2:1 ( I think. I know its in the low 8's) that is why cadillac requires plus fuel in it to keep pinging down to a minimum.
Like they ^^^ said. The version in the Cadillac (and also in various Silverado models now, both the SS and in others as the "Vortec Max"), just uses a higher compression ratio for the added little bump up over the 325hp in the other version, which also makes it require Premium.