If I understanding you right, you're asking if you can get the grades by question category in addition to the grade for the whole test. Is that right? If so, I had the same question a month or two back and although I didn’t get an exact answer, I believe from the responses that Moodle 2.0 does not allow for this. (I hope I’m wrong)
What I did to work around it was to build my quizzes in smaller sections by topic and set it where they have to finish the first one before the second one is active etc. This way I have a better idea on the student’s weak spots and areas they should focus on. Although it’s more quizzes they have to take, this works for me for now.
I would like very much to know that Moodle 2.0 (now 2.03 for me) has the ability to give grades by question category and that I’ve missed something, so I will be watching the responses. I’m not a Moodle expert, just a relatively new user still trying to get it all figured out.
Hope this helps