I think that the offset is just what you suspect - more efficient blade entry into the water.
I've had Fenn 1's in the past that had offsets of between 6 and 9 degrees and a SET Lettman that had a 5 degree offset (the SET was obviously made from the same mold as the Fenn 1). A difference of 1 degree was very noticable, surprisingly. I preferred the Fenns at about 8 or 9 degrees. The SET Lettman was horrible, absolutely unusable. It felt like a tree branch.
More recently, I demo'd a prototype wing that had a normal or conventional shape. I thought that it suffered from too low an offset when I was using it for a few hours (on separate occasions) at a length of 209. Several months later, I tried it at 214 and the offset felt fine. This makes sense because a longer paddle will form a more acute angle with the water if placed next to the ski at the catch. So, shorter paddles will feel best if they've got more offset.
Can you have too much offset? I'm certain the answer is yes, but I don't know what that feels like.