Hi there!!
the rondo movement for the Mozart piece is grade 8. the best thing to do is ask your teacher but if you can play that piece well then you could take grade 8 if you wanted to. not many people jump straight in at grade 8 but you could have a go. also you should look at the grade7/8 syallbus, get some pieces from there and play them!!! then you are playing more different types of pieces.
you have to remember that if you decide to do grade 8 there are loadz of other things like scales and aurals and sight reading besides the pieces so you will need to work on them as well!!
hope that helps and makes sense!
Nat