From the little bit of experimentation I've done, KF8 formatting does not work in the iOS app with side loaded content (even if delivered via personal documents service). There was some discussion of this on eBook Ninjas podcast:
http://ebookninjas.com/2013/08/16/ep...8-iris-febres/
Apparently there is some iOS issue that Amazon has not been able to resolve with Apple.
There is some level of support when the content comes from Amazon, and side-loaded fixed-layout KF8 seems to work.
My takeaway is that you cannot test directly, if it looks okay with previewer and the other apps that may be the best you can do for now.
You might also post to Twitter with hash tag #eprdctn, I'm sure other people are dealing with this and might be able to help or confirm what the issues are.