If nobody answers you here then let me know. I started a group for phonescoping on Facebook back in 2014 and there are several folks that phonescope with the Kowa and Iphones. I can't specifically say the 5S version but I know they use Iphones.
I used the 5S for about a year and a half with my Opticron scopes and I was very satisfied with many of the images that I was able to take with it.
I used the Kowa adapter with an iPhone 4S with decent results. The main benefit I found was the ease and speed of use. It's much lighter to carry around and the workmanship from Kowa on the adapter is excellent, and of course it fits very well on the 'scope.
I've since changed my phone to a Nexus 5X and I bought the Kowa adapter for the Samsung S5, which I then "modded", i.e., hacked parts of it off, to fit the 5X. It works really well, again I don't have to carry another device and I can easily slip it onto the 'scope when needed. I've also used it with my Zeiss bins for some decent record shots.