Taking the grade 8 exam will definitely challenge you. I know the rockschool grade 8 makes you learn the half diminished scales, all major and minor scales, arpeggios up and down (everything 2 octaves). It really does help in the end as you have a new approach to playing and see things in another perspective.
Pieces themselves are also challenging if your not familiar with the techniques such as slapping, tapping and so on. Again it's a learning curve and helps you branch out your musical interests.
But again, it's a piece of paper saying you've achieved grade 8. You can't turn up to an audition and show them that and expect them to hire you Lol !!! (I know someone who did that)