I've had the Tres Leches at Portos along with pretty much everything else they make. It's pretty good and definitely worth bringing to the potluck. I've had better Tres Leches but it's usually homemade or when I'm in Mexico. The other thing is that Portos has built such a "legendary" reputation (not sure if it's that warranted) that if you tell people you got it from Portos, most people walk away satisfied.
IMO, I would check out Casa Bianca in Eagle Rock (1650 Colorado Blvd, Los Angeles, CA) and The Kitchen Italian Cafe & Pizzeria in Old Town Pasadena (78 West Union Street, Pasadena, CA). They are both terrific and thin crust.
Kitchen Italian Cafe & Pizza