i know i'd have to fab up some engine mounts and pull a rwd bellhousing out of my ***, but overall i think a 650hp 2.4 litre could really increase the power to weight ratio and also help save gas.
Your not going to save any fuel by installing a 650 hp 4 cyl. All your power is going to be way up top. Now if you went with a 250 to 300 hp turbo build and keep your foot out of it, you may get better mileage and it would be much more streetable. I don't think you'd want to let the clutch out at 5K just to pull away from a stoplight.