I weigh 200 pounds and I commute 40 miles to/from work every day with my '05 DR200. With the stock rear sprocket and no engine mods, I found that it cruised happily at 42-45 mph. Sure it could
get up to 60-63 mph on the highway, but it buzzed like a damn saw and the vibrations made it feel like it was going to explode at any moment. The sweet spot where it could cruise all day long without pushing the engine too hard was definitely just over 40.
I just replaced the rear sprocket with a 40-tooth from Rebel Gears. There's a 4-month lead time on the steel 39-tooth, so I went with their recommendation and bought aluminum. It may not last as long, but they're running them on everything and they swear their aluminum isn't soft. Anyway, after 70+ miles I can say that it made the bike much more road-worthy. As near as I can tell, it adds about 12 mph to the bike and it feels like I've added almost one additional gear to the top end. I can cruise all day at 55 mph, and even when I push it to 60 I don't feel the vibrations. I'm sure my mileage will improve as well, but I don't have any numbers just yet. On the low end, I have no problems starting out because that engine has so much torque. It's a little like starting out in 2nd gear, but not quite because my new 1st gear still has more power than my old 2nd gear.....if you can follow that.
It's important to note, however, that my top speed on flat pavement is still around 63 mph because of the reduction in power. It'll go faster if I tuck or lose 50 pounds (my wife says just cut my head off), but that really wasn't my goal. I was looking for a way to keep up with traffic on some of our rural highways, and I got exactly that. I don't climb trees with my bike, so the "loss" of my lowest gear is a non-issue. I couldn't be happier with the results.
Now I just need a DR350/650 to cover the interstate and long-range touring, and then I'll be all set.