A number is more useful than "the latest". (It may not be by the time people read this.) There can also be newer versions, or "hot fixes", that do not show up if you check for new firmware in the browser interface.
It might also help if you told people what the modem is in front of this router, if there is one. The make and model number could be useful. Is it, by any chance, also a router, with a set of LAN ports on the back?
The reason for asking is that a lot of people turn up here trying to put a router behind a modem that is also a router. That can complicate troubleshooting.
Are you testing with the same Wifi clients on the same network and with the same test site?
but you didn't specify if you were testing over 2.4ghz or 5ghz?
2.4ghz is a slow network and 1/3 the speed of a 5ghz would be considered fast for 2.4ghz.
Also, try swapping the 2x R7450's 5ghz bands. The lower 5ghz channels are low powered channels and will drop of speed quicker than the higher 5ghz channels. It might just be the fall off from the lower to the higher bands you're seeing.
