I started it but stopped after the summer because i was the only person who wanted to do it and the school couldn't find the teaching time for it, but to convince me doing it wasn't a good idea i was shown stats that showed that it must be incredibly difficult as the pass rate is so low. Also, around only 250 candidates were presented for the exam in the whole of Scotland, don't know if you can read anything into that or not.
In my school last year 3 people sat and only 1 passed (with a C).
If you're gonna get a C in higher, i don't think AH would be a good idea. Even the parts i did i found very difficult. A big jump from the higher course.
Don't know if what i have said is helpful or not!