You get a tiny fraction of the power of the card though, a 1080 would get you the performance of a 750. Also, I think it's kind of insane that instead of just getting decent onboard graphics your option is to buy a whole nother graphics card, power supply, and the adapter which can cost 500-800$.
If you are serious about graphics you'd probably just build your own desktop. Desktops have the best graphics cards and every hardcore gamer I've seen doesn't use a laptop.
Not a hardcore gamer (obviously being a Mac owner) I just want something that at least in Bootcamp is reasonable for the few games I want to play. WOT is one that works OK running under Crossover. But the really cutting edge games require a bigass video card you need a custom PC rig to play. If I bought one of those I'd get too little done :-)
No you don't [1]. You get within a (reasonable) margin of error of full performance (assuming there are no CPU bottlenecks).