Every Carnival ship I've saled on has had such wonderful food in the main dining room that I've not tried the extra pay restaurants. I did try the ones on the Emerald Princess and can attest to the fact that the portions are so huge, I know you wouldn't want another lobster tail. My hubby had the lobster in the Crown Grill on the Emerald and it covered a serving platter, it was huge. He didn't get it with a steak, just two huge lobster tails.
Be sure to try the chocolate melting cake, at least once, it is awesome! We're doing the Mexican Riviera in December. I, too, have no clue it Mazatlan will be there or not. Although I'd like to experience the "Love Boat" itinerary, it will not hurt my feelings if the port is cancelled.
Even on Carnival's smaller ships, like the Jubilee and the Holiday, I didn't think the lines were that bad. Like Jim said, at peak times, they can be a bit of a wait, but I've never stood there for more than 15 minutes. We try not to eat when everyone else is, but sometimes you just can't avoid it.
I honestly believe you will enjoy the Splendor, she's a beautiful ship. We've not sailed her, but have seen her in ports. I believe she's about the same size as the Valor, which I sailed last May. If it's the same layout, which I think it is, she's very easy to navigate.