I own it.
I have US copies of both films separately as well as the Japanese boxed set. It may not be worth it buying the boxed set if you own the films already. Cause it has the separate versions as well as the full film as one. But it was an afterthought for me and living in Australia, I didn’t get the chance to see the double bill together like it was intended, so I jumped on the DVD when it was released in Japan.
It’s a complete set, but it really just depends on how much you care. It might not be different enough for you to want to buy the same films again. But just to let you know, I own two copies of True Romance, two copies of Reservoir Dogs and two copies of Kill Bill Vol. 1 among other things. So I do have a tendency to buy an improved version of a film and then keep my old copy. I am kind of wasteful like that, but I can’t help myself sometimes.