I like the rubber grommet idea. Less mess than tape.
An engine tuned for maximum fuel efficiency is also tuned for burning valves and pistons. Pick your priorities with care.
The 49-state TW carb tune is perfect for absolutely nowhere under 6000 feet altitude. Every stock TW I've ridden is hard to cold start, takes forever to warm up and run right, has poor off-idle throttle response, has a lean surge in 5th gear at a steady cruise of 35-37mph in cool weather, and the engine runs hot. If this sounds good to you, keep the stock tune. If you'd like easier cold starts, faster warmups, better off-idle throttle response, smoother mid-range cruise, a cooler running engine, and more power, go one size bigger on the main, put a thin flat washer under the needle circlip, and open up the pilot screw. Of course, you'll lose about 2.145678901mpg of fuel efficiency. The bike will still be a bit lean at sea level and return good efficiency, but should be spot on heading up the mountains where the grade can cause over-heating with a lean tune, to slightly rich at 6,000 feet, where you pop the grommets.
Barring major changes of 10 teeth or more, changing sprockets isn't going to affect fuel efficiency too much. 14/50, 14/47, 15/50, and 15/54 yielded about the same efficiency for me. 14/54 and 15/47 both saw efficiency slip a bit. I'd stick with the stock sprockets or 15/54 if my mountain roads were paved, 14/54 if dirt.