I took my 5-speed Mirage from Ohio to North Carolina and back. I too was concerned about the high RPM, both for the potential harm the high RPMs might do, as well as the noise. I have no long-term proof that the high RPMs don't damage the engine, but the engine noise didn't bother me at all.
From what I've read, the 3A9X engine the Mirage uses has been in use since the 2003 Mitsubishi Colt. That's a good 15 years old, and I'm guessing that any major longevity issues would have certainly reared their head by now.
View my fuel log 2015 Mirage ES 1.2 manual: 44.0 mpg (US) ... 18.7 km/L ... 5.3 L/100 km ... 52.9 mpg (Imp)