There won't be any.
The problem the DECC have with trucks is that whilst stationary, they may pass their noise emmisions, but when rolling, they become alot noiser. It's not just about compression braking, there's road noise, engine noise, they pick up the trucks quite easily who have leaking exhausts a LOT easier then employing 4 guys to wave every truck in and go over it at an inspection station.
Owen, you can't hear much of the trucks on the other sides of those walls. You'd be VERY surprised how good they are at taking out noise. If there's a worry about compression braking annoying neighbours, they put up signs stating "Please limit compression braking"
When coming down the Mt Ousley, the trucks aren't on compression brakes the whole way, that's why they're limited to low gear. When you come down there in a truck, you actually use the throttle coming down it. Until you drive a truck down there, it's hard to understand...
And Terry, there won't be a difference at all, they're both picking up noisy vehicles, as stated, it's not JUST for compression braking.