I find the Chao take longer to level up in SA 1 than they do in SA 2. I mean, in SA 1, I only had one Chao, and it took me a long while to get through all the races. By the end of it, most of it's stats were in the L40-L60 region.
On SA 2, I found it a lot faster to level. Behold my Dark/Running/Running, L99 in every stat Dark Chao. And I had a lot of others beside this one, my Hero/Flying/Running chao did quite good for the Hero races. Didn't max him out, just got him high enough to get the Hero chao emblem. (around L60-70 in most stats).
Yeah, I did a lot of breeding, but why not :-)