Is a 12 second, 20+mpg slant six possible?

The '03 to '05 SRT-4's were capable of both with their 2.4L turbo 4 bangers and some upgrades.

So I'm with Dan. Yes, very possible. How much money do you want to spend? A well built turbo /6 with EFI and an overdrive gear would do it no problem. You could probably make it happen with less than that if you build carefully and tune well, but that would be the sure bet.