Welcome to the site. The Matrix/Vibe is a good car. I had one and did a few things. The wife (mainly) and I averaged 36 mpg over the lifetime we had the car. My build thread is here:
http://ecomodder.com/forum/showthrea...read-1890.html
I didn't have too much done to the Matrix, but it still gave good mileage.
The grill block is a must. I kept my lower one open in summer, and closed it all up in winter. The scangauge is a great investment. The other things will yield less of a return.