For the processing that needs PRIME, OpenCL is indeed not used. This doesn’t prevent its use for other processing without PRIME that may happen in the meantime though.
Basically a recent (< 5 years old) discrete GPU is indeed needed to see a benefit with OpenCL. I don’t know about power consumption as it’s hard to get the information, but there’re many great graphics cards below 300W TDP.
Also I can’t tell more for now but please note that for the next version of PhotoLab, having a good GPU will make a significant difference.