The only time I go though the hassle of mitering borders is when there's an obvious stripe or repeating print. I agree with the other posters that the print is too subtle and the miter isn't going to show. On the other hand, if you want to practice mitering corners, this is probably the perfect fabric as any errors are not going to show.
I LOVE your quilt. Those colors are just wonderful. As far as the border width, I personally would add a 1 to 1.5" dark brown (or rust) stop border first, to really frame the top, then add as much of the leaf print as you need to make up the rest of the 6 inches.
As far as the additional cut for the miter, I'm kind of in the middle. I usually cut about 4 or 5 inches extra. You will cut it off in the end, but I find a little extra fabric makes it easier to do.