If you have a laptop with decent specs, I know you already suffer from the mobile version of the GPU. I will present you my setup, and what problems to expect when you take the risk to do something like this for yourself. I will make a video about this when my friend gets out of the hospital.

Laptop:

Laptop model: Acer Aspire V3-772G, 17.3" , full HD CPU: Intel® Core™ i7-4702MQ 2.20GHz GPU: -dedicated - GTX 760M 2GB RAM: 16GB, 1600Mhz

External Setup:

Adapter: PCIe PCI-E V8.4D EXP GDC External Laptop Video Card Dock / Laptop Docking Station PSU: OCZ CoreXstream, OCZ-CXS500W GPU: Radeon RX 580, Saphire Nitro+ Limited Edition

This is an extended subject but this is worth it if you don't have the money to build a pc with a better CPU than the on in your laptop (my case).

I will leave a list with the most important things to consider. For any details you can leave a comment.

Make extended research about your laptop's version of bios. Usually you have to deactivate the dedicated video card from there;
Go with AMD not with nVidia. AMD scales a lot, and I mean A LOT better than nVidia, and it's probably the same thing as with mining. The adapter is pciE 1x, so the GPU needs RAW POWER and shit tons of memory to run well. Besides that, nVidia knows about this way of using a desktop GPU for a laptop so they've canceled it from the drivers. If you buy nVidia, you will be stuck with a very old driver. if you go with nvidia, AVOID 1000 SERIES.
some open world games have hickups (the only I've found is farcry4 with this kind of problem, but Rust and Assassin's Creed run very well)
Even tho with AMD it's mandatory to have an external monitor, The nVidia isn't better. If you think about it, you have a 1x lane to transport data from the cpu to gpu, and from the gpu (with the hdmi cable) the images go to your monitor. If you use the internal monitor of your laptop with nvidia, that 1x lane cuts in half, one for sending one for receiving, and you get even lower speeds.
There are some stability issues which I suspect are from the hdmi cable from the pcie slot to the adapter.
Be careful what kind of PSU you choose... This model from Saphire eats a lot of power when running full speed, so when it loads, or when you use the menu from games you get coil whine.
Make sure you fixed your card in the adapter's slot correctly!!!!!!!
The performance loss in games, with AMD is between 5% and 20%. With nvidia starts from 15% up to 35%.
If you don't have an core i7 on your laptop, the bottleneck will be huge.
Be careful when you mod your bios, back up your windows. Something happened, I don't know what, but the system blue-screened when I booted on windows every time. I did a fresh install and everything worked fine.

I made this list because I consider you would have an advantage if you plan to invest in something like this...and again, THIS IS NOT FOR MINING THIS IS FOR GAMING!