When I got gr5 I had 98%
I looked at gr 6 papers and didn't even understand the questions
Yes, it is a big jump.... I had a really good teacher. We used the workbooks for 6-8, the pink and blue abrsm theory books (but not much), also looked at extracts from scores to help with identifying things like harmony and period.
I thought I'd never get there sometimes but it was worth the effort in the end, I got distinction on all 3 grades
If you want to do it the I say go for it!! But I think it would be very difficult without a good teacher.