+1 for frame of mind and attitude.
Most miles on a day on my KLR was 300. It was directly after I bought it, and I hadn't yet sorted out all the things the PO had left for me.
Worn out Shinko 705's, stretched chain and a 16T sprocket. My hip joints were a little sore, but I hadn't ridden any bike at all in over a year. I later lowered my footpeg 1" to cure the sore hips. I averaged 60-70mph that day. That was also the day that I decided that the KLR didn't live up to the bad reputation I'd been reading about. I remember getting around 58mpg that day. Bone stock bike other than the 16t sprocket.
The bike wasn't riddled with vibration. At all.
The seat wasn't absymal. In fact, I love it.
The motor didn't seem taxed. At all.
However, stretching those extra 10mph to cruise at 80mph? Yeah, you'll turn the motor a little harder. I'm now on a 14t front sprocket. I turn 70mph at a little under 5k rpm quite regular. Not daily, but a couple times per week, 30min each way. The bike does fine and actually travels with less throttle rotation than with the 16T front sprocket. IMO, the KLR doesn't have the power to pull a 16t at high speeds. The engine spins slower, but you are working it harder with much more throttle input. Sometimes I find myself accidentally running 80 when I want to run 70 (65mph posted speed limit). I figure if the bike can accidentally do that, I must not be straining it too much.
Have I ridden better higway bikes than a KLR? Yep, no doubt about it.
Can a stock KLR live running 80 all day? I think so.
Would I buy a KLR for dedicated highway duty/commuter purposes? In town, yes. 100%. For higway? Yes, unless someone builds a road bike with enough room for a larger guy, then no.
The ergos of the KLR fit me good. All other bikes feel cramped to me. And I've tried a bunch. I'm comfortable, so I'm hinging on KLR's motor to live up to its bulletproof rep to keep it alive. It's only been a few months now, but I'm very happy with my choice.
I ride 50/50 off-road and on. Love my KLR. It's a very basic machine with no frills or creature comforts. Either it fits you, or it doesn't. If it doesn't, they'll be miserable. If you need creature comforts on a bike, you'll not like a KLR. Jmo.