Ok. I grabbed an image to test. This was a purposely underexposed shot as part as a bracketed set. In the images below, I tested pushing shadows and highlights to the max with the RAW and the Jpeg in Lightroom. Then, to further beat up the images, I ran each through Nik HDR Efx. I'm showing shots from each step as well as some crops. Because I quickly did the processing, this will be my first time viewing the comparison.
Straight out of camera jpeg original.
SOOC Jpeg processed in LR
Raw file processed in LR
SOOC Jpeg (Dark image above) run through Nik HDR Efx
SOOC jpeg processed in LR then processed in Nik HDR Efx
Raw image processed in LR then processed in Nik HDR Efx
100% Crops...
SOOC Jpeg
SOOC Jpeg processed in LR
Raw file processed in LR
SOOC Jpeg (Dark image above) run through Nik HDR Efx
SOOC jpeg processed in LR then processed in Nik HDR Efx
Raw image processed in LR then processed in Nik HDR Efx
So the obvious take away from this? Raw is better for this type of extreme recovery. However, I'm pretty impressed at how well the Jpegs held up even after being run through two destructive processes. What I've found on previous work is that minor sharpening with masking, clarity boost, WB adjust all seem to have minimal impact on the OOC jpegs.