Can I substitute Kale for Savoy Cabbage in soup receipt?

Only if you like kale. Ick

I still don't understand the kale fad. I have no idea what your soup recipe is but I see no reason why kale couldn't substitute for the savoia. And remember there are varieties of kale, including "curly" kale and cavolo nero (black kale). There was a time when curly kale was used strictly as a garnish and nobody ever actually ate it.

The flavors are so different. Kale stronger, savoy cabbage so gentle and sweet.
I would not substitute. Find a recipe that calls for kale.

Yes. It might take a little longer to cook and has a more pronounced flavor. A little vinegar is good with kale, depending on what else is in the soup.