Yea you basically got it covered you just need chain cassette and really just the right side shifter you can re-use your left one since its the same either way, but as far as the X-9 in 8spd I have never seen one. No new hub is required.
There are several places that sell XT in 8spd, but my suggestion is to email SRAM about it, since I figure you are running a SRAM rear derailleur you probably can't use the XT. But in all honesty an 8spd won't really do anything for you as far as an "upgrade" it truely isn't much lighter and the weight since the chain is thicker, also finding a good 8spd chain is a bit harder than finding a 9spd chain.
I am running an 8spd set up, at no point do I feel when I'm riding like I need 9spd, but when it comes time to get replacement shifters/chains/cassettes I sometimes wish I did since right now many of the parts are much easier to get a hold of.
If you simply want less gears since you don't use them, losing one gear in the rear really won't change much, save your money and invest in something else IMHO.