I’ve been charting my sales since I put up my eBooks in May 2012. I have sold 4x the amount of books on amazon Kindle than I have anywhere else. I have my books on Smashwords which distributes to lots of place, then separately I put them up on Barnes & Noble and Kobo Books. For every 100 books sold through Amazon Kindle, I sell about 15 at each of the aforementioned places.
So, it makes you wonder if you should just forget about the other places and concentrate on Amazon. The only time I did KDP select was with my first book, and really, you need other books in the wings waiting to hit when you put a book in KDP.
I wonder how it would be if you put all your books for the 3 months in KDP? I now have 8 romances published. Would it be worth the experiment (and the time involved!) to unpublish everything for a 3 month period from everywhere except for Amazon Kindle? And then put all 8 books into KDP, and stagger the free days for each book. So that is basically 40 days of free books on KDP, each title getting its 5 free promo days and hoping that each of 8 book gets into the top 100 at least once in the 5 days.
Food for thought. I’ve actually never heard of anyone doing this. It would be a cool Russian roulette experiment, minus the bloodshed of course.