What makes you think that 480 Defense is the cap / point of diminishing returns? From the graphs and such that I have seen on the various forums regardings stat caps and the points of diminishing returns, it seems that a defense rating of slightly over 600 (about 30%) is where diminishing returns kicks in.
My Guardian has Defense Rating of 606 (29.79%). Still working on getting my Shield Chance and Absorb a little bit higher but that would be easy if I could determine that Defense Rating's cap was 480 and I didn't have to stack it to 600+.
Here is where I got the information regarding the stats:
Immortal|Guardian PvE End Game Tanking Compendium - 1.2
Scroll part way down for the graphs and such.