Hi, @jersmi. Sorry I didn’t get back to you sooner.
I don’t think there’s a practical way to keep the size intact pixel to pixel (and eliminate the black border as radius increases) with the current blur, right?
We’ll try to add an option for that in the next release. In the meantime you could calculate the inset rectangle and use the Crop Image Pixels node.
To verify – should this method work with layers, i.e., Render Layers to Image?
Yes, it works for layers, too. I just posted an example: Feedback Stripes.