parts for jdm costs more
labor might be same, i cant say for sure as not experience with city
mileage depends on a lot of factors
engine health
driving style
filters/oil being used
traffic conditions
etc etc
Mira is quite fuel efficient car(one of the best in 660's), if everything is in good nick you should be able to get 20+-
they are fine to drive on highway if you drive sensibly and dont push them to limits
like they are small car built to be efficient, treat them like that and they are great
talking from personal experience of move custom 2011 & 2013, mira 2013
edit: join the group mentioned, you will get a lot of useful info here