I second the positive aspects of 15t front sprocket.
I do mostly highway (100km/h) and drops your rpm by 500.
Im tempted to get a smaller BACK sprocket to get even lower rpms at high speed... but I would have to go with either 41t or even 39t... meaning i would probably have to shorten the chain. I have some hills over here and Im kind of weary of having to drop to 5th or even 4th with more frequency. Shortening the chain to try it out, however, is a nono.
15t on the front is a very easy mod (except for getting that damm nut off, check the threads for tips on that). It gives you a smoother ride at the top end, a little increase in top speed, and not so much power loss at the bottom end.