In my personal opinion, its better write pure native app for Windows Phone 8.
You can also checkout whether you can use Aviary SDK(http://www.aviary.com/w8) along with Rhodes Native extensions.
PS: This is my personal opinion and not RhoMobile's
We decided, we would rather have a native app or using a cross platform framework that allows to create native apps. Hence we won't do more investigation into RhoMobile and will close this thread.