roho - There are only 2 minor issues I find with using the Mirage for long trips. Both of these 2 issues stand out to me, as I also own an LS 460L, and it excels on the interstate. And that thing is a half an acre of car, 385 hp, with possibly the best sound system going (especially for a 2010).
1. Gusty winds blow it around. But nothing terrible. I notice that when I get blown around, I look at the car behind me, and it too gets blown around. Just not as bad. This adds just a little bit of effort to steer back toward the center of the lane. Nothing major. But what do you expect from a 12 pound car with a 6 inch wheelbase?
2. There's enough noise coming through the tires, suspension, chassis, body that lowers the fidelity of the music being listened to at interstate speeds. At least with my ears. I have some tinnitus. Mine is weird, there are noises I can't hear, and noises I do hear. People mumbling, I don't hear much of what they say. However, I can be walking in my house and hear a faucet dripping inside. It seems like I can hear things that represent a waste of money, where my wife can't hear those things at all. However, she can hear a flea fart from 100 yards away. The point is, you can hear your music for the most part, but it has to be turned loud. And if the trip is long enough, your ears may ring a while when you get where you're going. Of course, this is my experience. Other people with really good hearing may not experience what I do.
One other item, which I think is the opposite of an issue. MPG. Sure, when you do 75+ mph, the mpg will drop. But if I were to do 80 mph, I'd say I'd get maybe 37 or 38 mpg. That's still if WAAAAAY better fuel mileage than my LS would get (maybe 21 or 22), or my Sierra would get (maybe 14 or 15 mpg), or what my Ram would get (16 or 17 mpg), or what my Scamp would get (0 or 1 mpg). So I think the high speed mpg is the opposite of a problem. I think it's GREAT.
View my fuel log 2020 Mirage ES 1.2 manual: 42.3 mpg (US) ... 18.0 km/L ... 5.6 L/100 km ... 50.8 mpg (Imp)