The oem speakers while containing an immense amount of excursion and relatively decent frequency response are only rated for 15 watts rms at 6ohms.
You will have to push ANY headunit to get them to be really 'good' speakers.
In my opinion as far as stock speakers are concerned I think the stock head unit will do the best damage. It and its amplifier were designed with those specific speakers in mind. I do not see you getting anything better with a head unit alone. The speakers will probably have to be changed. I haven't messed with my stereo because I intend to trade up my mirage eventually but that is my opinion.
View my fuel log 2014 Mirage ES 1.2 manual: 36.2 mpg (US) ... 15.4 km/L ... 6.5 L/100 km ... 43.5 mpg (Imp)