So -- pros and cons of real meat or shellfish as an occasional treat?
And if okay, raw or cooked?

IMO I would stay away from any seafood. It can be an allergen and it's
not something cats would eat in the wild. Boneless, skinless chicken
breast, cut into chunks, is something I give my cats on occasion. You
can cook it (plain) or give it raw. Use organic meat if possible so it
doesn't have any additives, etc.

Rabbit is another option (organic if possible, also), raw or cooked.

Do NOT give raw pork. There's a risk of pork having trichinosis (sp?)
and passing it along to the cats.

Another option is to buy chicken Liv-a-Littles, which are freeze-dried
chicken breasts. Most cats LOVE them.