Well, Before you take eleven of the unofficial CATs you should try out some different options, and you should maximize the number of official GMATPrep tests that you take. I have never heard of GMAT Premier so I have no comment on those exams.
So, yes it is worth it to purchase the two additional official CAT exams from GMAC to add to the GMATPrep.
Another thing that you should consider is broadening your group of exams. Rather than taking 11 exams from a those two companies why not try the the FREE practice test from Veritas Prep. This test has shown a close connection to the scores received on the GMATPrep and on the official GMAT exam.
For access to the FREE Veritas Prep practice test
https://www.veritasprep.com/gmat/free-gm ... tice-test/