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...