I'm definitely going to put a similar question to, Dr.Gunson, see his opinion on the matter. I've been recommended SARPE + 3 piece lefort by two different surgeons as I have a narrow palate, personally, I want my maxilla expanded to enable enough room for my tongue to rest, as well as widening the nasal cavity, because at present my tongue rests on my bottom teeth which just exacerbates my open mouth posture.
Common logic, supports what Mew/Mckeown say with regards to tongue placement in the roof of the mouth. From what I've heard 2/3 of the tongue should rest in the roof of the mouth.