Streetable 12 second 20+ mpg slant six?

Seeing as you posted in the forced induction forum I am going to assume that you mean with a turbo or a supercharger. In a word yes it can be done. Best bet is probably a turbo but it won't be cheap unless you are real creative. 12's with a turbo won't be to hard, just pricey. Talk to Prine, he will help a lot with a turbo setup.

Jack
Talk to who???