Never heard of these until reading about them on a Hyundai forum. Check out Rigid Collar.com - Subframe Rigid Collar Kit - Improve your Vehicles Handling - Honda | Nissan | Toyota | Mitsubishi | BMW | Volkswagen.
Seems that just about all mfgs., don't know of any exceptions, use very generous mounting holes in their subframes, through which bolts are threaded into the body structure. The slop in the bolt/hole clearances is supposed to be resolved by heavy torque on the bolts. Apparently, the subframes still move about in their mounting holes. Certain aftermarket manufacturers have made fitting collars for the subframe bolts so that the bolts then fit snugly. Reviews of these rigid collars have been very favorable.
And, it seems the rigid collars are fairly cheap and easy to install.
Anyone know of applications for Jaguars?

 

You would think that being a UK based company, if there was a benefit to installing them on a Jaguar, they would have such an application listed.