Yes, I found this article on the subject. On the bottom of page 3132, Graph D appears to show 100% survival at 10 years for AA patients with the trisomy 8 mutation:
http://bloodjournal.hematologylibrar.../3129.full.pdf
Also, it mentions the mutation may be transient as Hopeful heard at the AAMDS presentation. It seems perhaps the trisomy 8 mutation in AA patients has a better prognosis than a patient initially diagnosed with MDS who has trisomy 8.
..of course, not being a doctor, I am reticent to fully trust my understanding of these things. But between this and the great info that Hopeful posted, it certainly seems like a question to raise with your niece's hematologists.
On the other side of the argument would be your niece's youth, and the promising advancements being made in MUD transplants. There is still risk, but much less than in the past, and there is a very real possibility of a full cure. This is only anecdotal, but I have seen some wonderful results of MUD transplants for SAA at my hospital, on folks much older than your niece.
I wish your niece the best in making this difficult decision. She is fortunate to have you!