I've had braces for 5 years and got them age 16. I have a pushed back mandible, narrow top and bottom dental arches, and I have proper tounge posture (I've had proper tounge posture since God knows when)
I seriously think that it works both ways... If you undergo FP, NCR, Palate Exoansion without proper tounge posture, you will not receive optimal results. Now, in my case, I feel like since my tounge posture is good everything should go great and I shall receive the best results possible since tounge posture seems to be extremely important. I don't really think that you can get around your entire problem if you're not addressing EVERYTHING. Practicing proper tounge posture without fixing a narrow top or bottom arch will just leave you disappointed & vice versa.
CP: Did you expand your bottom arch considerably width wise?