I would suggest getting a LAN wire connection to the xbox.
However if the new settings and such are working for now, see how it goes.
Ya, if yo use a lot of other wifi sources using the same channel, I'd see if you can change the xbox to use 5Ghz if the range is short and kind of close.
I tested my xbox on wifi and worked well however router and xbox were with in 3 feet of each other.
After testing, I've gone back to LAN wired.
Something else to consider, might look into some DHP-701 PowerLine units between the router and xbox. The 701s are Gb supporting so should have good performance handling for gaming.
Any chance of someone putting in some CAT6 lan cabling in the attic?