Google tells me that Saigon Cinnamon = Vietnamese Cinnamon, which I use for cooking. IMO, good spices are wasted in soap - I buy from Dollar Tree. I really think the subtle differences that make this better than other cinnamons would be lost in soap.
As far as ground cinnamon in general in soap, I like it. It can be an irritant, but so can lots of things. I like the look of it, and it lends a noticeable aroma to the soap, though not nearly as strong as an FO or EO. If I decide to rebatch a bunch of different-colored scraps, I add ground cinnamon and clove because it makes the brown soap look better/more interesting.