Bus Man is one of those subjects you don't have to spend much effort on (at least on my part).
It'll always scale down heaps. But, for one thing, if you're interested in it, and can apply examples, you'll do exceedingly well. Apparently, if you lost 5 marks in the exam, that's still a 50 provided you have A+ SACs (anyone care to confirm?)
Regardless, it has scaled down less in recent times. But, don't count on it becoming scaling neutral. Such a popular subject being done by... not so good students will bring the subject down.
Remember, they pick a study cohort at various schools and compare their SS's in other subjects to determine BM (i.e. they get 30 in BM, but average 27 in other subs - BM goes down to 27.) It's more complicated that that, but it's a general idea.
My school has been part of the cohort in recent times (I am under the impression my CLASS is IN the cohort, which is good for everyone). But such is the size of the BM group, we exert little influence when we pull ~40's and everyone else does 25-30 SS's (thus bringing BM down proportionally).
EDIT: various schools, not one school.