Hey, welcome to EM, fchen7!
I'm glad you find my Vibe mods inspirational!
I never bothered doing an "A-B-A" test for the grill block. It can't be much, I would guess around one, possibly two mpg. It's just one small mod that when combined with several others, add up, you know?
Yup, just bought an Insight to commute in. But we're keeping the Vibe. The wife will have the Vibe to drive during the day, and it will be the family car (like it has always been) on the weekends. I've taken the pizza pans off because they were so rusty, and the wife has requested that I leave them off.
I may paint up a new set that imitate the "cranked" five spoke alloys and see if she'll put up with that. She's pretty agreeable. Usually.
But I would be happy to make a tracing of the grill block for you. Send me your address in a Private Message and I'll get it to you. I could even include one of the lower blocks that I run in the winter.
I don't want to discourage you too much, but you're facing more of a challenge with the GT. It's equipped with the 2ZZ engine (as opposed to the 1ZZ-FE of the base model) and the 6-speed transmission which has a shorter 5th gear. This will work against you to some extent, but you should still see some decent numbers with a few mods and some informed driving. Check out
the 100+ Hypermiling Tips link, this is where you'll see the largest gains. There is no mod, aerodynamic or otherwise, that will improve your MPG's like adjusting your driving technique. We call it "tightening the nut behind the wheel".
Good luck and have fun!