I have followed this topic out of personal interest so I suppose my conclusions might be helpful.
I would say that I can see the following pattern for Set(O) applicants - we now have a few cases of people who applied in August, went for appointments instead of using the app (never used the app) and got their decisions in two-three working days time (myself included). On the other hand there are certainly no cases of August applicants here who used the app (never went to appointments) and got decisions. In fact there are no cases of June or July applicants who just used the app and got approval so far. Hence we can safely say that those August applicants who only used the app will probably have to wait for a few weeks. I am pretty sure that the reason is that the picture submitted via app has to undergo additional verification and for some reason this process takes time at least for now.
So personally based on the data we have so far I would still go for an appointment now if one is available within the next two weeks, if cost is not an object. This situation might change in the near future but we have no data to tell us when or if indeed.
I do these type of analysis for a living so just my two penny