The Star Wars Trilogy is a synthesis of Westerns, Serials from the 30's, the films of Kurosawa, Ww2 imagery and ideas/films, and many others. The presence of Shogun influence is not a surprise, nor does it mean that it is the chief influence in the trilogy: it, like the other influences, makes up 1/100 of the whole.
Read this:Show me where the idea of "son redeems father", Lucas's obsession with limbs getting cut off, man falls in love with much older woman in a forbidden relationship comes from... I'm not claiming Lucas took ideas from the East and Westernized them. Star Wars is his own creation. Just that, why does Avatar follow the story of Pocahontas so closely?