Lower gearing WILL help ... but nothing will bring back your Sea Level HP riding up over 6500 ft. ALL bikes lose power at altitude. Physics.
A couple ways to help:
Leaner jetting could be helpful if you are now running TOO RICH. (a quick look at your spark plug will tell you: black plug = too rich) So get your jetting right first.
Also ... adjusting the Low Speed Fuel mixture screw will help a lot with low speed running, including allowing smoother running, better throttle response, and better starting and idle.
I believe you can install an "extended fuel screw" on your Carb. These are easy to reach and adjust. Idle speed should be adjusted too. Changing low speed fuel mixture takes just a minute with the extended fuel screw. XT Vendors must sell something like this?
If you go back to Sea Level, be sure to change Main jet back to a larger size
(richer), then re-adjust low speed fuel screw (takes 20 seconds) and idle for Sea Level operation.
Two ways to affect gearing.
1.Rear sprocket change
2. Front sprocket change.
I don't know stock sprocket sizes on your XT but most likely a SMALLER front sprocket is available. As mentioned above, ONE tooth smaller on front sprocket equals 2 or 3 tooth difference on rear sprocket. It may be possible to go 2 teeth smaller on your XT ... this I don't know ... but for sure One tooth smaller should be possible and WILL make a nice difference.
In you case, riding up SO HIGH, I would DO BOTH. Go up two or three teeth LARGER on your rear sprocket and go ONE tooth smaller on your front sprocket.
This should help make the bike more ridable and help keep you out of that low RPM lugging zone. (never good on a single cylinder bike!)
Your top speeds on highway running will be limited just a bit but overall performance should be better and ... MORE FUN!