The clonks, or the curing of them, was one of the reassuring things which the dealer did to make my purchase seem more worthwhile. He didn't say that the clonks would return as soon as the belt went slack or that continuing to run the belt tight enough to stop them would wreck the rear wheel sprocket.
What the clonks are, in reality, caused by is the winding up of the NVH kit which, when you go over a bump, unwinds suddenly to bring the bevel box down with a clonk against the chassis. Tightening the belt pulls the bevel box back and up at the front and so relieves the problem but only temporarily.

2013 M3W 1960 Velocette Venom and the Landrovardo!