8 is a good score - I would be happy with a dog with a score of 8. :-) In breeding terms you are looking for a score below breed average if possible - all other things being satisfactory of course. :-) And in a pet - shouldn't be any probs in terms of HD with a score like that. :-)
At the moment, we are saying in the breed that a score under 14 is acceptable. My Aussie has a score of 8 too... he is 3:5 and that is absoultely fine. Have you passed his score back to the breeder ? I am sure they will be thrilled with that. If the dog is who I think he is, his sister has just come back at 8 too :-)