I haven't done a ton of research on this, But before I moved away from Apple last month I did some digging on band-aid patches to keep Apple in the conversation for new media servers. From what I read, you have to do some behind the scenes coding to only support the external GPU via something like the Razer Core. BUT without ANY warning, if an update gets pushed through, even something minuet or minor, it can overwrite what you've programmed and or completely brick your machine.... Again, just what I've read, haven't tried it. For the money I found getting a Laptop with a GTX 10 series would end up saving me money and time.
From what I've been told there's been issues with the GPU switching and that's why people have tried to hard-code the external GPU as the only one used. Seems like a lot of work, but if that's what you're wanting to do, I'd love to see the outcome.
My only complaint about moving back to windows, is that i love the os of Apple better, and I really miss all the syphon apps available, Spout has about 1/10th of the available apps as syphon.
With that being said, the hardware and price point of the current Razer line, or any other competitive "Gaming Laptop" blows the current MBP's out of the water.
Also, The stuff I was reading about was on the trashcan Mac Pro's using a GTX 1080, Not sure if there's any differences between that and the MBP's. With the os being the same, I would assume that the process would be the same.