hi, Anamorphic.
There are plenty of Guys who are using external GPUs on this forum & those run great. Computer sees PCIe cards as if they were physically inside of computer.
Performance (rendering speed) will stay the same & the only difference is going to be upload time, tat will be influenced by connection speed - lane account, PCIe gen =)
On the other side 80W is not so much.. at best You can power up is 750 (Ti). Recently
ASUS release interesting SKU based on 750 - STRIX, having not 1 or 2GB (like most of 750s), but 4GB =) - the only problem here, even if You can power this thing, You might end up having to tear Your nice box into pieces..as physically it would be hard to put that card inside..
so what's the solution? well, to buy a bigger box, with more power & be able to house bigger cards, or..choose this route ( I should get my hands on it in the end of this month & I'll share my experience if You're someone will be interested =)
Keep in mind that connection speed for this kit is even lower & power brick is limited =) but You can buy it without it & choose regular (ATX) or SFX power supply (recently those come up to 600W) - so there are options.
P.S. if You want box for Your new parts - there's a user in the forum with milling machine =) maybe he could help You to make custom piece so You can fit Your chosen parts into nice package =)