A Selmer S80 is probably not the best of pairings with a Z. The problem there is that the Z is an open horn and the S80 is not an "open" mouthpiece. As far as legit mouthpieces go, I can't say much. I got super lucky with mine, but that's a story for later. A Z will likely respond better to mouthpieces with a more open chamber than an S80 (esp. one with a big E tip). Even a Larry Teal will probably put you in a better spot as far as long-faced legit pieces go.
I'd go for something with a big chamber, slightly under-cut walls, a medium facing length, a tip opening close to your S80, a long roll-over baffle, and rails that aren't too thin. Work up to a hard enough reed to cut out that "reedy" sound and get a ligature that blows freely enough for you to support a great tone at pianissimo.
I'm never a fan of metal pieces on alto. I don't play a style that calls for it, though, so there's that. Most hard rubber pieces will do well for you and bother the neighborhood animals a little less. For legit, hard rubber is almost a definite. Don't be afraid to try jazz mouthpieces described as extremely (or whatever adverb you like) warm. These will be better for switching from your jazz piece if you're a jazzer, and sound way better than you would just switching to a closed up thing.
The G1 neck is a known tuning blunder in the Yamaha sax world. A G3 will be better in tune, but a little more restrictive. The V1 is supposedly a better blend of open and in tune. The only neck I've really put through paces is the Oleg. I like it a LOT. There are many other brands out there. Just make sure there's a return policy and to take what you read about them with a grain of salt. Also know that many of the biggest players you've heard of use Oleg necks and have for years. Oleg even used to make a neck with the Selmer insignia on the front, so you can't always tell on YouTube videos. It should be obvious why that's not an available option anymore.
I did try a Mk. VI neck once on odd advice that originated on SOTW. Apparently, it's a good idea for Mk. VI and EX players to just switch necks, because there's a group of people that swear each works best for the other. It sounded pretty nice, but the bore size wasn't exact and it wasn't as good as my Oleg.