My 2017 G4 heater is quite sufficient and I find myself turning it down from time to time as it gets too hot if I have it on manual. I have the automatic HVAC so mostly I set it at 72 and forget it. It also warms pretty fast. I have the heated seats which are wonderful, they warm fast too. A friend has a Spark with heated seats and I can say the Mirage seats get warmer faster and hotter than his Spark. The high setting is almost too much and I rarely if ever use it more than a couple minutes.
Enjoy your Mirage!