I know you developers are accustomed to seeing nothing but problems posted here, so let me tempt fate, risk the wrath of the computer gods, and cross my fingers, while saying that I tried Spacedesk and a few competing commercial apps, and Spacedesk is working much better than anything else I have tried. It is a small install, provides many options (even for for the HTML5 viewer running on my 2011 27" iMac) including a full 1440 resolution, and has for me, worked beautifully. In fact, I prefer running Spacedesk on my iMac than using a directly connected 24" 1080 monitor as a secondary display.
I am looking forward to buying a license when it eventually becomes available, and am wondering how much better it could work with a native Intel Mac viewing app.
Cheers to you all!