I can't speak for America, but last year I did several thousand miles on what you guys call two lane black top, I think. I had the lowest powered bike in the group, and all were impressed with the S40's ability to keep up in the 60 to 70 mph range. Longest day was about 400 miles. At 75 mph the bike will be starting to work hard and according to the experts your oil consumption will rise. You will probably want a windscreen for long trips, but not if you're an iron ass or hard man. You need a break every 100 to 130 miles and your small fuel tank will enforce this. My Air Hawk seat pad helped a little, maybe a lot. Much of the riding was hilly to mild mountain. We don't have "super slab" where I live, and some on this forum don't consider the S40 to be suitable in that environment. I had my pack mounted on the pillion seat and this formed a semi useful back rest.

http://beatduck.blogspot.com/
Here's some good reading for you.

It should handle 70 with no problem. Most I've ridden in one day was just a little over 240 miles. That was at 70 & 80. Wasn't very comfortable, though. The stock seat sits too low for me. I modded mine with a contoured piece of 1" thick rigid foam between the stock foam and the seat base. I'm going to add another 1/2" next week to give me a little more leg room. It not only raises you up but you can sit a little farther back in the seat. Here's the link on modding seats.
http://diymotorcycleseat.com/

I am female, but weigh 170 lbs.  I owned a Savage in the late 1980's (first bike I learned on). I recently bought an S40.

The stock seat on my S40 makes my butt go numb pretty fast.  I had purchased a better seat with sissy bar for my Savage and took the Savage on long rides.  I found the more comfortable seat was much more enjoyable.  And I could strap packs of stuff to the sissy bar. This was before I bought a set of saddle bags.

Interestingly, I didn't use the saddle bags that much, as everything I needed was around my waist in a fanny pack or strapped to the back of the sissy bar.

Either bike had no issue accelerating on the freeway and I have done 80mph on both bikes, with plenty more to go. About 85 is the max I ever had either bike at, I could have gone more, but just wasn't interested.

My longest trip, by myself, was San Diego to El Paso.  Now, that was a ride to remember!

First thing I am saving for with my S40:  A much better seat!

I've been just cruising on an almost daily basis, and just pick a point and ride. Generally that point has been around 300+ miles away round trip. That said I do some 70mph but prefer to turn off and enjoy random backroads with GPS sort of rerouting me when I am "lost".

All of that to say the bike does well for me. I enjoy it's limitations because they take me on journeys I wouldn't otherwise be on.

Rejetted carburetor, stock muffler, BIG windshield, and my 6'2"+ roughly 250# self will hit 80+ riding to work each day. Atrociously pathetic fuel economy, went from 68mpg down to 48mpg.

I hate commuting on the baby bike. I've put just under 5000 miles on it since mid-April of this year. I'm on my second back tire, it needs front brake pads, needs a new oil filter... but does not need an oil change, it does that all on its own, I go through a quart or better a week high speed commuting in 90+ degree, 90-100% humidity.

I did have to switch to premium with 100:1 premix 2 stroke oil added to keep the pinging under control.  And I run straight 30W oil, 20W50 full sump is dry in a couple days, 10W40 in under 40 miles.  

I may be a bit rougher on my late model toys than most other owners are... I'm replacing my 20 year old mini truck that couldn't hack it with a 45 year old full sized truck that can, and has, survived my ownership.  

I take route 50 to visit friends in DC and Baltimore, and drove up to Maryland from Georgia on 97 last fall (was going to school in Savannah, originally from Annapolis). Cruising at 65 and speeding up to 75 for overtaking is smooth. But, at 5'4 and 130 lbs, windy days are absolutely terrifying for me. Apart from eating up oil, it's how light the bike feels that makes me uneasy at speeds over 65, not any kind of mechanical issues.

If you have a mill, or a friend that has one - the conversion to use a Kawasaki front pulley is a big plus when you do lots of highway traveling.     It changes the front pulley from 23 teeth to 25 teeth, and it cut the rpm down 8%....which is a worthwhile change and well worth the effort if you have the ability to machine the Kawasaki pulley.

You can also do the rear pulley - but for a stock engine on a bike with luggage it may be geared too high to be ideal.  At the moment I am the only one I know that is running the double Kawasaki pulley set up, and it is working great on my bike.  It does move the axle all the way back to the end of the adjustment for belt tension - and I don't know if that becomes an issue with a high mileage belt.

http://suzukisavage.com/cgi-bin/YaBB.pl?num=1384949185

Dave