Stewer, you're a genius.

Using a math node worked beautifully. Here's the full scene. The ground mesh is 24,000 polys and covered by a single 4096 square pixel map, same lighting and render settings as before. I did a bunch of test renders to get the scale to match from both maps - the value in the displacement was left at 0.035 so the only change to the material screenshots I've already posted was the addition of a math multipy node with the 1st value left at 1 and the 2nd value at 155 for the exr map. Rendering the same image as posted before they both looked the same on screen, so I zoomed right into the model (the area circled in red) to see whether an extreme closeup would show if there was any more detail being carried in the 32 bit map.

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