Finally got to checking out Update #2, and the new Quad Warp effect.
The good news: The corners track better than before, which makes a lot of difference.
The bad: The image is pretty soft. Could there be an option to dial down the anti-aliasing a bit while still tracking the corners better?
The Plugin version looks sharper and I don't think it ever had the wandering corners issue in the first place. Nope, just checked: has same wandering corners issue, more visible at lower res, or when following something. Added an image of it doing it in a plugin. All 4 corners start and end at the same coordinates following a box moving across the screen and expanding slightly. (Another plugin issue is it doesn't accept small enough numbers to be placed at pixel boundaries, just 0.00 to 1.00 values to represent X and Y coords.)
Bug: There's still that weird render bug where it grabs pixels outside the texture, producing lines at the edge.
This test showed a white line periodically appearing at the right edge of the texture.