Because of the very simple optical design of the diopter (or, technically more correct: close-up lens), the image quality will suffer a lot, e.g. from heavy chromatic aberration. The results will be like using the cheapest, crappiest lens instead of your quality Sigma one.

Instead, I'd recommend an extension tube. IMHO, they are a much better value for the price: better image quality and much more flexibility.

An extension tube will not interfere with the optics of your lens, just use it a bit outside of its designed distance range. From my experience with other lenses, I'd expect quite decent results from your 56mm prime.

A 10mm extension tube, used with your 56mm prime, will give you a focus distance range of roughly 20 to 30cm. And they often come in pairs or sets, so you can achieve various extension values, meaning even closer distances possible.

And they only have to match your body mount, and can thus be combined with whatever lens matches your body. The close-up lens, on the other hand, can only be used on lenses with the same filter diameter.

When shopping for extension tubes, just make sure that they are "automatic" ones, connecting the camera signals to the lens, and that they have a decent mechanical quality (e.g. looking at user ratings in online stores). My set for a Canon EOS-M was in the $20 price range, and I'm completely satisfied with them.