I am currently using AIO --> Z2 which looks really good on my red car. I had been using AIO --> SG --> S100, but this gets me a very similar look and takes one less step. Not to mention that Zaino is much more durable than S100 and at least for me, a bit more durable than SG.
While topping a synthetic with a carnauba has the advantage of bringing out a bit more depth, I prefer being able to add new layers of synthetic without stripping the carnauba, so I've simply stopped using carnauba, at least on my red car. It still looks a bit odd on our metallic black car, so it gets AIO --> S100, skipping SG or Z2. Not that AIO isn't a synthetic, but it doesn't give the same appearance as either of the sealants.
If you're going to go to the trouble of keeping up a carnuaba finish, I'd give Pinnacle Sovereign a shot, it's a bit pricey but really brings out nice depth in red and other dark colors, especially in non-metallic paints. Of course you won't lose anything but time experimenting, I haven't tried topping NXT with anything, so go ahead and try 16 over it on a panel and see if you like the difference.
I like to try multiple things on different panels just to see how they look and how they hold up. It's also a good way to find out how much combinations dust up and deal with water spots. Have fun!
Ben