Are you able to get above 200 up wired? I had the same exact issue. XT8 connected to FiOS ONT, only able to get 200 up. I gave up and am just using the XT8s in AP mode with a FiOS G1100 router I got on eBay.
I had an issue where the 2.5 GIG WAN port was negotiating a weird speed with my FiOS ONT. for me this manifested as slow upload speed on wired and wireless. I would also see CPU usage on the router go way up. My fix, after going back and forth with ASUS support for 2 weeks, was to put a switch...
I figured it out! Looks like the issue is with the 2.5 GIG WAN port and the connection it negotiates with the ethernet port on my fibre ONT. I put a switch in between the ONT and the WAN port of the router and now I am getting full expected wired speeds...
HA sorry I missed that and then re-read your post right after I replied. Is anyone else having this issue? I'm assuming it must be some kind of NAT acceleration issue.