I would leave alone if its not leaking at the joint. Braze joints are stronger than what you think, and as the UFM is such a large well built and strong construction it won't weaken it. As for cosmetic appearance, once the fuel tank is on you won't be able to see it at all. You could smear a thin coat of filler over the braze joint before painting, but I would just paint it.
Cheers,
Simon.
Cheers,
Simon.