Grades are completelysubjective and not that important (I pay no attention to them).
I am often shocked at the ABRSM grading of pieces. Last year, for instance they said Brahms waltz op.39 no. 9 was grade 5, the same as Schumann's Fantasie tanz (op. 124 no. 5). I would have graded Brahms at grade 3 and the Schumann at grade 7. Then, back in 1966, Satie's gymnopedie was grade 3, but in 2000 it had been promoted to grade 6!
Besides, there is no great difference (imo) between grade 6 - 7 - 8. Anyone who can play a grade 6 piece can tackle a grade 8 piece.
As for the Schubert and Mendelssohn - they have very different technical and musical requirements, so they may not be comparable at all - I find them both equally difficult (or equally easy) but very different pieces to play. Anyone who can play one of them will be able to tackle the other.
A much better comparison would be between Grieg's Arietta (much easier) and Mendelssohn's, since thery are very similar in structure.
Do not worry too much about grades. They are completely unimportant.
Best wishes,
Bernhard.