I have been able to recreate a couple of issues or maybe they are all symptomatic of one issue.
1) the brightness of the backplate in reflections is brighter in XPU - see mirror ball in top image 2) the CPU render (bottom image) gets a more accurate reflection (maybe in this render it looks a bit dark..) however the shadow is too bright. The XPU shadow looks better. So neither is giving a good result. If I combine holdOutShadows from XPU and beauty from CPUthen all good. 3) With this scene if I have the second input connected to the karma renderSettings and vars of from 2nd input is ticked my scene is very unstable. Especially if I am on CPU in the view port. I can get renders of both out to mplay no problem but often my houdini session will lock up if I am on cpu and then try to switch to xpu. My scene is generally stable if I stay off CPU or do not have the second backplate connection in place.
Ok, so first thing I see is that the color of you backplate (by the name of it I understand it has been somehow taken from the rendergallery) seems to have its colors off (that may or may not be a houdini issue). I would try to render out the backplate images through husk to make sure the colors match. This should get you there with CPU. On the XPU renders it appears that XPU renders reflection on the reflected backgroundplate-shader coming from the lights in the scene which makes it brighter and looking more correct (which it is not). I am not sure if this is what one wants from the backgroundplate-shader to be honest. So that might be a case for an RFE. In the link below find those two images (for the domelight and the backplate) with matching colors (and for the domelight.exr re-exported from nuke because hdr files can be a bit wonky with colorspaces).
I did rotate the domelight to y 90° So I can see the reflection of the sun move and see if the shadow of the sphere on the ground is reflecting correctly. so for you to use those maps, you need to rotate the domelight as well
