Modern Vintage Gamer has just posted a video demoing the original Xbox’s System Link feature working with the game Crimson Skies. He was able to run an original Xbox alongside an Xbox 360, Xbox One and Xbox Series X also running the game, but in their backwards-compatibility modes. With all connected to the wired LAN, the game seemed to work fine.
While Xbox backwards-compatibility isn’t perfect, Microsoft should definitely be commended for their efforts. In my very little experience testing original Xbox on the Series X, I found that the games I tested loaded fast and looked great. It’s a bit frustrating that you’ll need to fill up your Series X’ SSD in order to play them, but if that’s what’s required for the performance boost to work, it’s a fair tradeoff.