I agree with a lot of what DamnSkippy says about embracing innovation. If you tuned a big (or even moderate sized) V6 as hard as say, the 2.0 liter I4 in a Lancer Evo (286hp out of 2 litres is 143 hp/liter. At that rate, a hypothetical 5.7 turbo would have north of 800hp), you could get some really outrageous performance. With that said, in the Challenger, as retro as it is, I would only want it with a large V8. If they came out with a different head design that made more power but couldn't rightly be called a hemi, I would be OK with it, but part of the reason for getting a car like the Challenger is for the sound and torque of a large V8. The future of performance (leaving alternate propulsion methods aside) is probably smaller displacement, forced induction engines, but the Challenger is not futuristic, it's retro. So in general a 400hp turbo six is a great engine, it just doesn't "fit" with the Challenger (for me anyway).