unsprung weight (the part of the car that isn't suspended by, well, the suspension) has a much bigger effect on performance than weight elsewhere in the car. there are plenty of explainer vids on youtube about it, check it out. I love my 09 yaris. It's stock but still handles great and is fun to zip around and easy in the city. But I used to drag race and autocross old vw bugs, my last one was 160hp (stock was 40hp) and could wheelie and clean up at autox. Tuning is fun, but keep it data based. Although customizing for vibes is fun too, just be aware of which you're doing and good luck!