Also, can you displace the collision mesh?
I don’t think you can displace the collision mesh with a material since I don’t think you can apply a material to it. You could use complex collision where the actual visible mesh polygons are used for the collision but I don’t believe that will work either because with vertex displacement the vertices are being moved on the gpu side after & seperate from collision calculations. For example if you displace vertices outside the object bounds visibility culling won’t take into account the new mesh bounds - so if you displaced them a lot you would need to manually set the object bounds larger or the object would pop in and out when you were looking at the edge.
It is very cheap though depending on what you plug into it - if you make some super complicated calculation that drives it that could get expensive but for anything normal it should be quite cheap and the cost should be reflected in the stats window of the material under vertex shader I believe.
I hope that helps.