I'm neither a teacher nor a professional. I just happened to complete both LT and FT by self-learning.
There are plenty of choices on the syllabus already. A candidate won't get scored higher because he/she is playing a piece (or more) that is on the syllabus of a higher diploma level. On the contrary, the more difficult the piece, the riskier as the candidate may make more mistakes.
Your pieces by Beethoven, Rachmaninoff and Debussy are quite difficult already. Why make your programme very very challenging. Remember the marking scheme : technical 30%, musical 30%, communication 30%, presentation 10%. I would focus on musical and communication more. Remember the exam fee is not cheap (at least in this part of the world).
It is stipulated on the guidelines that candidate should present a balanced programme, and indeed I think you should actually consider baroque, american/latin american, spanish, or a very modern work to as your 4th piece.
Good luck.