I've played and beat both games. XIII-2 I just beat last summer, and I watched all the cutscenes from XIII just before playing XIII-2 again. I never really messed with the datalogs in XIII, but I never found what was going on too hard to follow. XIII-2 I did read through all of the fragment stuff. It had interesting tidbits but didn't add too terribly much to the story.

The point is I am pretty well equipped in my knowledge going into LR. The only thing I haven't experienced is the novel(s?) for the XIII series.