CineBench benchmarks show the quad-core Core i7-8706G to be just 9 percent behind the Core i5-8300H in the latest XPS 15 9570. This processor is currently not available on any mass market consumer laptop and can only be found on the Precision 5530 2-in-1 workstation for now.
https://www.notebookcheck.net/Rare-Intel-Core-i7-8706G-Kaby-Lake-G-processor-is-almost-as-fast-as-the-Core-i5-8300H.338748.0.html
Since the specs are the same for the CPU (except vPro) does this not mean it's the thermal characteristics of the laptops you tested that explain the difference?
I have Dell Precision 5530 2-in-1, interesting enough is that in ThrottleStop there is available overclock of the CPU +7, and I am able to set all core boost to 3.9 with no problem, with undervolt, result is stady 840 in Cinebench with around 85 degree on CPU.