So I’ve been doing some experiments with metallic objects to try and improve the quality and I have a few notes. The attached GIF demonstrates using the RayTracing Feature Switch within the material to better integrate metals into the reflected scene.
SSR: Just to remind us how far we’ve come 
Lumen SC: Reverts to matte base color. Not a great look.
Lumen HL: Much better and sharper reflections in general as expected but now our metals look black in reflection.
Lumen SC + Fallback Cubemap: Metals are starting to appear integrated into the scene reflection, but results vary highly on the quality of the cache, as expected. Good result.
Lumen HL + Fallback Cubemap: Reflected metals are now approaching the Path Trace in appearance, very good overall and not dependent upon cache quality.
A few more notes:
Scene Captures do not support Lumen. The fallback reflections would integrate much better if the global illumination was captured. In my example scene, the scene is too dark without GI. This is the most obvious difference between the real time and path traced scene. I would love to have support for Lumen in captures.
I found getting good results with the cubemap required using the emissive channel, the base color alone didn’t usually integrate well. However, there doesn’t appear to be a way to prevent emissive materials from casting light. This is what causes the faint glow on the bottom of the cubemapped spheres.
There also really needs to be a way to disable emissive materials from actually casting light into the scene for the sake of artistic control. In this case, I just reduced the strength until they seemed well integrated but not glowing.