I have a 2010 KTM 530 EXC. I have done a lot of the trips with Rsquared where he has been on his 450 EXC or his 690. I have never really had an issue with running highway on my 530, even without any kind of aftermarket windscreen. We usually run 70-75mph when we hit the highways. We've done stretches of highway riding that have been several hours in length at a time. I would not say it is fun, but for me it certainly isn't miserable. The seat I have sucks. A new one would be nice. The bike handles fine. The biggest issue is that I have to pack LIGHT. The 690 can definitely carry more. Fuel range is not an issue for my 530 or his 450 because we have the 6.6 gallon tanks installed, giving us a range of around 300 miles on a tank. There have been times in Utah and Montana where that was a nice thing! The last trip we did to Montana in July it was not a problem for his son riding the 690. His BIL had to stop and do the RotoPax thing somewhere out in the middle of Wyoming.
As always, it REALLY boils down to intended use. When we get in rough stuff, the 690 can do it, but it just takes a bit more effort (unless you are Rsquared's son, in which case you rip through it like it is nothing...). While I can do highway miles on my 530, that is not its forte. I prefer to use it for trips that will be predominantly dirt. That said, we did a trip to North Carolina where we had our "big" bikes (1200 GS and 990 Adventure) and our small bikes (450/530 EXCs) and on the twisty pavement, I think we had more fun on the small bikes, even running full knobbies! We have done quite a few trips where we trailered to our destination, taking big and small bikes for each of us. Then we ride them on alternating days on bike appropriate routes. That let's us take a "relaxing" day on the big bikes after each day on the small bikes. This works great if you base out of the same location and do loops each day. It also allows us to run knobbies or an aggressive DS tire on the big bikes without worrying about using them up getting to wherever we are going.
For an extended road trip where I might hit the occasional dirt forest service road or county road, I would take the 1200 GS in a heart beat and put something like the Heidenau K60s on it. They will last MUCH longer than the TKC 80s I prefer for more aggressive dirt riding on the GS, and they do pretty well on everything except mud and deep sand. On trips like these, we might do several hundred miles of pavement in a day and the comfort of the GS over the 530 makes a big difference. On the trips like the last Montana trip, we were doing anywhere from 200-300 miles per day with a mix of pavement and moderately technical dirt. I could have done probably 80% of it on the GS, but that other 20% would have REALLY sucked on the GS
Like Rsquared said though, the vast majority of the bikes present on that trip were 690s and 701s. Surprisingly, there were very few KLRs. There was a time when it would have been the most prevalent bike on these kinds of trips.
The riding I consider to be technical is the stuff that usually has you doing relatively low speeds because the terrain is just rough, exceptionally rocky (as in BIG rocks), very steep, lots of deep ruts, deep sand for extended periods, muddy, tight narrow tracks, downed trees across your path, rocky ledges you have to get up or down, etc,... It is these conditions where the weight of the bike makes a BIG difference, as does your ability to get a foot down if needed. The other end of the spectrum is pavement and maintained unpaved roads. It is almost impossible to get a single bike that excels across the whole spectrum. It is always a compromise. My 530 covers the rough end well and the middle, but not the easy end. My 1200 covers the middle and easy end fantastic. The 690 seems to cover the middle and a little of each end really well.