I've had my Gryphon in my 04 for about 2 weeks now and it has to be the best investment I've made. I'm running on Level 2 and the firmer/quicker shifts and better throttle response makes it a whole new truck. I can't say I've seen a change in my mileage. I'm still getting 15-16mpg.
Still waiting on my custom tunes from Bill, I'm getting anxious