I watched the video 3 times then went in to his channel.Found the headers 1 7/8 inch for forced induction and off road pipe your right.No cats here in illinois wont fly emission test every year.Impressive power gains with that set up though.Again original question was how much power will e85 add to a bone stock 5.3 over 87 octane im still going with nothing except worse gas mileage by at least a few mpg.I ran it in my 2012 silverado bone stock for almost a year when it was $1.75 a gallon and gained nothing that i felt but never dynoed it.
I have no figures for Gen III FlexFuel motors.
For Gen IV FlexFuel 5.3, with Sierra or Silverado tuning, they go from 315 hp @ 5200 to 326 hp @ 5300 and torque from 335 @ 4000 to 348 @ 4400.
For Gen V FlexFuel 5.3, the L83 makes 355 hp @ 5600 and 380 @ 5600 and torque from 383 @ 4100 to 416 @ 4100.
You can see that the Gen V has better tuning.
My Yukon XL makes 307 RWHP through the traps in the quarter mile on E85 @ 73% and has a Magnaflow Y-pipe and Cats and a BlackBear tune.