Since fitting my XT clutch mech over a year ago I don't think I have lost the chain once on my 2 x 10 setup.
For this reason alone it has been well worth the money but as far as I can tell its done absolutely nothing to quieten down my bike.
Just a little confused why there are so many people claiming clutch mechs reduce or eliminate chain slap.
Having looked at the marks on my bike from where the chain hits, it appears that the bit of chain that flaps about is the top line rather than the bottom line. It makes sense as the top bit of chain doesn't look like it can be effected or tensioned by the clutch since its isolated from the rear mech at both ends by the chainring and sprocket.