I actually started out with using uniform colors, shapes, etc.

I can only give my own experience, but I find that those earlier images are universally harder to remember, even when I don't have the image in front of me and I'm just trying to recall the set on it's own. This is true even for cards where I have only four items in the set for the uniform images, and upwards of 15 for the non-uniform ones.

I think that what happens is that these extra cues help in the initial learning and memorization. As I get better, I can simply visualize the location of the node in the image, visualize the attached image, which brings to mind the text. I have trouble getting to this point when I don't have the other context cues to help me out initially.

I don't quite understand what test you're suggesting in your last paragraph. I think what you're saying is try to memorize a random set using simply text, then a random set using simply the images, and then test myself outside of anki by trying to recall the sets. If so, I have done this, and the images (with the crazy shapes), outperform by a large margin. I can't remember a set of more than about 5 using simply text in Anki.