I tend to agree that adding another fletch isn' t going to help much. I think it' s generally accepted that you are exceeding the practical speed limit for fixed blade heads unless your bow is about perfectly tuned and your form is right on the money. Could be, also, that your FOC is tad low and you already have too much drag on the rear of the arrow.
I think I' d be tempted to try 3 four inch feathers and see if that helps. As much as I despise all you long draw guys ( envy) there are times when my little 27" draw and 270fps comes in handy.

Barry