im left handed and i use mirror mode very rarely, needless to say it really matters on the map. i will say that my left hand is far superior then my right in about everything, including osu mania, so if there's some part of the map that's very intensive on the right hand, my right hand seems to just lock up and not be able to press anything, causing me to tank my accuracy. in maps with heavy right hand bits, mirror is very useful for me.