I haven't looked into it at all, but I hear the new 6R80 transmission is doing great in the new 5.0s. I'd love to have a couple of extra gears just to help in around-town everyday driving, along with extra performance at the track, plus the reduction in drivetrain loss. However, I can't imagine it being worth the cost and hassle just to gain that little bit of performance.
And also, you spoke of fuel mileage. While you may gain a little from reduction of drivetrain loss, you wouldn't see anything from the overdrive gearing. The 4R70W (what you have now) has a .70:1 OD ratio, while the 6R80 has a .69:1 OD ratio. In other words, no noticeable difference.