I fitted the stoppers in mine a few weeks back when I swapped the engine over prior to the Bulldog drag strip in August.
Wasn't sure that they'd work, but on last Sunday's Essex/Kent Storm meet mine was getting increasingly rattly, and on stripping it that evening I found that the spring had snapped, but that the stopper had prevented the CCT from retracting far enough to allow the cam chain to jump sprocket teeth.
So that meant that all I had to do was replace the CCT with an old one I had lying around, and make a new stopper for the replacement.
Done in an evening, instead of the two days it takes me when the valves bend and you have to take the head off.
Saves the cost of valves, a head gasket and the shims.
So once again

YES


BTW, it still did 110 on the M25 away from the tolls at the Dartford Bridge (lovely little blast up the hill when the barrier goes up) so I would think that the stoppers would carry on working for a fair few miles.
Not sure I want to check out this theory for long, though....