Well, first answer this: does your current motor run well?

If the answer is yes, then I wouldn't bother. The d15z1 is so gutless it actually hurts mpg because you have to run it harder just to keep up with traffic. I honestly think a z6 or d15b is capable of just as good mileage. d15z1 + Si/EX tranny might be tolerable, but you'll still have to keep it slow on the highway. :/