Yeah, "essentially the same game" is hyperbole I've been seeing all over the place. Sure it's similar - it's a sequel, not some "sequel in incremented number only" like the majority of the Final Fantasy series.
But it is different where it counts - much larger, much more complex map. Entirely different story, with the main commonality being the Arisen's origin (which is the point of the title, so shouldn't be all that different).
It's improved all over the place,aside from the performance of course.
I would have been pretty happy with another content DLC for the first game, so even if it was genuinely "essentially the same" I would probably play it.