You've got to see the sort of questions asked before you take the test and there is no better way than to look over past papers. Most centres stop teaching theory after grade 5 and I'm not surprised since the difference in the upper grades is like that between GCSE and AS/A2 level. Grades 6,7,8 Theory are progressive and really hard. Not for everybody, so see what you're getting into by looking at a past paper or two.