Hmmm... Interesting...
Over the years, I have had several students who opted to play Mozart (1st or 3rd movements) for their Grade 8. Whilst I understand what the others are saying regarding popularity and the examiners having their personal opinion etc, etc, I always advise the students to choose something they like, they enjoy, and the pieces which show off their strength.
Choosing Schumann is a lovely idea - I like these pieces a lot!

I hope that you have a competant pianisit to play with you. (Mozart has much easier accompaniment). Remember it is a chamber music, too.
Playing Mozart on any instrument brings a lot of discussion regarding style, articulation, expression etc, and undoubtedly, everyone has their own opinion on how to achieve the best performance. But surely, the examiners are open-minded and welcome performances which are well-prepared...
Best of luck with your forthcoming exam.
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