I've done a day trip into SAO but I had a later departure out of GRU (~1am). The bus wasn't too expensive and I was able to walk around SAO at my leisure. Got some good pictures of the cathedral.
Possible, yes. Easy, so-so. Safe, yes, but pay attention.
Worthwhile? I think so. I got to see a different culture for a few hours, eat a big bologna sandwich at the market, try cheese bread, etc.
MPN? Ahhh, I hope to make it there one day!