I have one of the prototype TBs from Doug/BilletFlow. It has been working flawlessly for about 6 years now. It's been so long, I forget when I actually purchased it. HA! HA!
Maybe 10hp or so with a lightly modded Terminator. When my old '03 Cobra was making 457rwhp the Accufab TB/plenum made 9hp. The real benefit IMO was the improved throttle response. I can't tell you what the increase would be for a ported Eaton setup. Probably not much more than 10-15hp.