So far this is what we have experienced with the two trickflow headed 408s that went across the dyno. One made 527hp/525tq and mine went 550hp/550tq, for referrence a a third 408 had cnc speedmasters and made right at 500hp. All three of these engines started out at or below 500hp with two having custom carbs and they all picked up quite a bit with a timing curve/carb work. None of them were full tilt builds with compression ratios from 9.8, 11.1 and 10.4 and solid flat tappets between high 240s to high 250s with mid to high .500s on lift.
