Check the UAI scaling info at
http://www.uac.edu.au/pubs/pdf/tablea3.pdf
I'm just looking at that info and telling what I saw:
* Compared to other subjects SDD isn't that poorly scaled. Apparently it's better than Geography, Senior Science, Standard English...
* If you do well and get HSC scores in the 80's or 90's, the scaling is quite decent.
Well to be honest when I was doing the HSC I thought SDD couldn't possibly give me a high UAI, because so few people got band 6. I ended up getting 87 for SDD and was surprised, it actually counted and I got 97.80 as my UAI. So yes, even though I thought it impossible, you can do SDD and get high marks!