Things to consider:
First of all you’re going to have to either dual boot or completely wipe steam os off your deck to install windows because cod doesn’t run on Linux. And that you’ll be doing yourself.
Second the performance won’t be that of your Xbox ( assuming you have one of the latest models )
You’re going to have debatable fps on bigger maps like ground war and warzone, and in modern warfare 3 I have yet to see someone play the multiplayer and get a solid 60 fps and the ones that do get close to 60 have degraded the graphics so low it’s not nice to play imo.
I played all of the mw3 beta period on my deck and I have not yet actually purchased mw3 yet because I’m waiting to see if the performance actually improves on deck before I do.
I have never recommended this before but if your sole purpose is to play cod I’d look at a legion go or rog ally. Don’t get me wrong the steam deck is better at everything else. But the other 2 already come with windows so you don’t need to do it yourself ( it isn’t that hard ). And they do have better performance.
That being said the Steamdeck is the only thing I game on and in fact it’s my only and main pc.
For some reason I thought he meant 90% res scale while locked at 30 fps ?