Hey,
This isn't exactly an answer, but rather a Q + A!
What an interesting piece of hardware. I've seen some vids and it seems to work well (although not in all cases). The concept is a great take on the very expensive units being offered by Alienware or whomever for their GPU docks. They seem to run in the 300-400 buck mark. This is a very cheap version, but very functional.
I'm interested, as I have a very similar laptop to yours. And have always wished I could game on it. This may be a solution.
I think it's a matter of selecting an appropriate GPU so it's not bottle-necked by the CPU and subsequently wasting money. Your CPU is decent, so it can potentially run reasonably mid-high end cards. The problem with a 1070 or Vega 56 is that the CPU will bottleneck those GPU's in certain scenarios, CPU intensive games, and most AAA titles coming out soon as an example. I'd be thinking of a RX480/580/gtx1060 for more balance (and being cost conscious). If you're playing on the Laptop screen then that should be plenty. If you use a high end monitor then the others make sense, but your still CPU limited.
Thank you for posting this. I've completely missed this simple solution. I'm determined to get one and try it out. My HD4000 laptop iGPU is muck. I have a couple of old GPU's sitting around (which still work!) and with a few cables, and extra power adaptor and some driver installation, i'm gonna have a great time setting this up!
Have a good evening (or at least it's evening where I am in Ireland)
edit: on the 1440p thing, i think you will hit the 40-60 fps mark, but you may have to manage in game details to reach the desired result, which is also game dependant. The cutback nature of the mobile CPU variants, mean they are just not the same as the desktop counterparts. You may find the CPU isn't good enough to drive those GPU's at such a relatively high res. At 1080p I'd say yes. But at 1440p might stretch the CPU.