In my experience an 8.1 does the same on fuel, so you can take that out. None of them are going to get better than 12 without a lot of very careful driving in a very light truck so I wouldn't worry myself about that.
The 8.1 has a lot of potential but it's limited by the throttle body. They just can't flow enough air to support the higher RPM. Also, the marine world has tons of horsepower parts but they say marine so they cost more, a lot more. They're a great motor if you leave them stock, with maybe a cam that emphasizes the low-end. But they can put a serious drain in your wallet if you get into them.
If I did a 454 I'd actually go back to a carb. The 96+ vortec doesn't make that great of power, and the tbi's are anemic. I'd build a vortec one and put a good sized carb on it myself.
88 RCLB: LS swap in progress
00 CCLB
03 Yukon Denali: Wife's "soccer mom mobile"