Is your XT Shadow Plus mechanism intact? The XT mechanisms have a design flaw and the strap clamp that acts as the friction damper can often break. Open it up and give it a look. Otherwise just tighten it.
As far as the effect of the cage length, you could assume that a shorter cage would need a mechanism that produces less torque because the lever is shorter, i.e., a similar chain tension from chain bounce would create less torque on the cage pivot. So assuming a SS (Shimano's designation for short cage) derailleur and a SG (med) or SGS (long) derailleur all had identically set Shadow+ mechanisms (same opposing torque), the SS would provide the highest resisting force on the chain and the SGS would provide the least. And thus with the SS, the inertia of the chain run would have less effect on the position of the chain run, i.e., it would slap less.
As a side note, I hate how everyone has taken to call these things clutches. They're actually brakes. They couple a rotating element to a fixed element through friction, which is the definition of a brake. Clutches transfer rotational power, but brakes dissipate it, which is what Shadow+ and Type 2 does. They're just brakes that only act in one direction. You could also call them dampers, because they damp the oscillation of the chain. And like most dampers, they only operate in one direction.
Oh and as a second side note, the difference between the FR and DH Zee derailleurs isn't the cage; they both have SS cages. The difference is the Bracket Axle Unit; they have a fixed tab that bears on the tab on the derailleur hanger, the one that a non-Shadow derailleur's B-screw bears on. The FR version has this tab more forward so that the derailleur ends up further back over the cassette, so it can take bigger cogs. The DH version is closer and more closely follows the profile of a road cassette but can't handle a large cog. The Saint Bracket Axle Unit has a screw on "Mode Converter" that does the same thing without requiring a completely different part, but this is more expensive of course.
