I've got a bike with Rohloff + Gates CDX belt in the back of the garage (it hasn't been used since I got my Mercury). Gates recommended a tension which resulted in a 50Hz frequency hum when the belt was plucked (Gates provide a phone app to measure this). However, I actually ran the belt a bit slacker (30Hz hum) without any ill-effects. However, that was still much, much tighter than Thorn's recommended chain tightness which is to be able to move the midpoint of the chain up or down several cm. I have to wonder if the belt manufacturers' recommended high tension is to minimise the risk of the belt coming off in use because, if it does, then it's likely to get damaged and new belts aren't cheap.