I was unfamiliar with the option to use Ctrl-L to “link modifiers” so out of curiousity I tried it out in the context of this problem. But unless I am not using it properly there seems to be no linking taking place. It just copies the modifier from the active object to the other objects in the selection.
No harm no foul right? Just add a driver to the value you want to be able to control globally by referencing a property on a controller object. However Ctrl-L to link modifiers does not copy the driver so this would still require you to go into each modifier on each object. So am I doing something wrong here or is that workflow not really as viable as you suggest it is.
Now, in general I think its a poor idea to use a decimate modifier to increase performance on high res cad models since especially on highly detailed models its slow as molasses. Its probably smarter to create a driver to toggle the max draw mode to bounding box or, if its neccesary to maintain some visual of the actual shape, use the remesh modifier. That thing is wicked fast compared to the decimate modifier while maintaining the original shape much better.
With all of that said in regards to the initial “issue” with the wireframe slider I think I understand its use better now. Its not necessarily a performance thing (anymore); it can be used as a visual aid with highly dense meshes when your working in wireframe mode.