Those features certainly offer places to drain water from the trail. I'm sure one of the attractions of a serious DH course is erosion and how to find a line through it, but in general water on a steep grade is going to be a problem and erosion is going to happen over time no matter how well it was made.
10% average is fairly steep, but the IMBA guidelines do not prevent sections steeper than that. Any experienced DHer is going to want some sections much steeper than that. Where there is bedrock, make it steep by all means. At the bottom and top of the steepest parts there needs to be maximum effort put into drains to prevent the track getting scoured out. In an ideal world the builder would allow the track to cure over many months before unleashing the hoards.
Interesting you said with our smaller hills in Oz. That is a problem. In the Alps you could get a 3km course with 20% average grade fairly easily, but here you would reach the bottom in maybe 1-1.5km in most places. Therefore the dilemma is length versus grade, which may mean features and manufactured tech rather than just a really nasty bit of hill. When you look at courses like Illinbah, Thredbo etc, the trail is not hugely different to a descending XC (AM) trail for the most part, but has sections of severe grade and challenge in amongst the faster, less steep stuff. However the less steep stuff is usually pretty rough, unlike a XC trail.
