Writing a paper on a smartphone implementation of previous work
Stepping away from the details of your particular study, what you are more generally asking about is the distinction between studies of theory and of practice.  

The first paper you speak of introduces a method and demonstrates that it can work, but does not characterize its performance.  The second paper would then be characterizing its performance in a particular environment of interest.  From a scientific perspective, that is certainly a contribution of new knowledge (even if incremental).

To determine whether it is of sufficient interest to be published, you need to focus on the question of motivation: who will want to know about the performance and why?  What else will they need to know, such that you can create a complete and informative study.  The answers to these questions will depend critically on the precise details of your work and its context.