Yes the 7th Gen 45W quad core CPU is a stronger chip, equally the performance of the new 8th Gen U series quad cores is not poor as some would have you think. Personally I doubt there would be any meaningful difference in performance, unless the usage is intensively CPU bound for prolonged periods.
Exactly. Kaby Lake R is optimised for burst, 45W quad cores are optimised for both burst and sustained performance. If all you need is burst (Office, everyday computing, Internet etc.), Kaby Lake R is incredible. Once you need sustain, its a significant downgrade from a 45W CPU. Of course, most reviews focus on burst performance (e.g. Geekbench). In other words, the Kaby Lake R is an ideal 'casual' CPU. And there is nothing wrong with that. I found it very strange though that so many people would consider it an upgrade over even a 7700HQ. Is it just because there is "Gen 8" written on the spec sheet?
If you need to run the system so hard, thin & light designs are never going to be the optimal solution, as by default their design reduces the cooling capacity.
The 15" MBP works very well demanding sustained CPU tasks.