The tagline "CAD for User Interface Design" did instantly catch my interest when it was on Kickstarter. As a former construction engineer I'm convinced that the usability for "power users" of a simple CAD like AutoCAD is far superior to any design app I know so far.
One of the greatest things of AutoCAD is, that you can click anywhere to set a starting point, then either change the current "New Element" or simply point the cursor in one direction and for example type 330 and enter and the for example a Square would go 330 wide and after that you could enter the value for the height. Similar were lines or Circles asked for radius or diameter. You could actually have drawn anything just by text-input by giving coordinates for the starting point. Alongside the far superior "snap-functionality" including helping guides and direction snapping or the throughly missing "tangential snap" in all design apps, AutoCAD had a far better usability. Of course this doesn't work for freeform, which in PS or AI is often needed, but for technical "designs" that build up upon simple forms like rectangles and circles, this is in my opinion unchallengeable and user interfaces often have that technical approach.
So if you haven't already done so, have a look at AutoCAD, I think it could be a huge advantage to Subform compared to other Apps, and be a great improvement to the current rather complicated way to create new elements.