Is it possible to concentrate on all of your organizations equally as opposed to concentrating on just a single one? I get that SS does indeed sell more, and that DT and FT are also in the top tier based on sales, however, if you take a broad based approach you cast a larger net, cumulative sales at other organizations can help to make up a larger sample size. And really, once you have your photos keyworded, with titles and descriptions, then submitting the photos to multiples stock sites shouldn't require all that much effort. Especially if you are using an FTP upload service that you can leave uploading overnight.
I am still experimenting with all of this myself. But I am thinking that the greatest loss of time is spent uploading and submitting your photos to these sites, so I am going to experiment with only uploading photos once I have reached a certain threshold - perhaps 50 photos - so that I can gain some level of efficiency in the whole process (keep in mind - some organizations like IS will only allow you to submit 18 per week and will inherently take additional time though you can use their deepmeta software package to make this process significantly easier).
Those are just my two cents. I am still learning and experimenting with the best way to approach this industry.