It would be my opinion that in the context of the carbureted engines found in the 1950’s XK’s, absent racing or very serious track use, you should see no benefit form this modification.
The tappet guides (the part that is pressed into the head) on Series 3 XJ6 4.2L engines were prone to loosening and floating upwards where they would come into contact with the spinning camshaft. The hold-down kits were developed to prevent this from happening. I have had a S3 XJ6 4.2L engine pass through my hands that had this exact failure.
The tappets in an XK engine are very slightly off center to the cam lobes. This offset imparts a force to the tappet such that it rotates a bit with every camshaft rotation. This ensures that the same spot on the tappet is not constantly exposed to the load from the nose of the cam.
Compared to the camshaft, the tappet guides are relatively soft. When the guides loosen they ride up with the tappet where their inside edge is struck by the spinning cam lobe. When struck by the camshaft two things happen: first, the guides begin to rotate in their bore, making them looser, and second the cam nose begins to gnaw at the inside edges of the guide. The result is a nasty ticking noise and a large quantity of very abrasive metal particles in the oil supply. If left unremedied long enough, you will soon lose a main or rod bearing.