The 5.7 tbi is pretty thrifty. I'd imagine you'd gain maybe 1 or 2 mpg at the most. Having that many more hp on tap at higher rev's may be too tempting.
I can consistently touch 20+mpg on the highway with my lowered 2wd 320k rcsb 350tbi/700r4 and 3.08s. I'd imagine 22-24 might be possible with 4.8 or 5.3. 3.08s are terrible though and I actually put them in on purpose for highway driving. Had 3.73s and they were vastly better on hills and take off. My truck went from respectably quick to average just with gears. My truck is now my project and I'm doing a 4.8 or 5.3 turbo and 3.42's or 3.73 are going back in. If you want max mpg get taller gears. Was good for 2 or 3 mpg in mine.
