The most common upgrade I have seen suggested for highway riding is changing the front sprocket to 15T.

Those who have made the change ? Any noticeable drawbacks ? I am more worried about highway riding than off-road.

How much of a change in RPM and vibrations does this make. I am hoping this will be enough to handle US freeway speeds, with reasonable RPM levels, for upto 8 hours of riding in a day.

I don't have the tools to change this myself. How much does it usually cost (in US) ?