if you can stack in enough electrical to handle or barely almost handle the pair of 18s then do them. that much cone in such a small airspace will slam - it could be a realley fun demo viechle
but if its your only viechle - daily driving to work and back grocery getter bla bla
then its not smart to turn it in to a all out ground pounder. you might end up needing better suspension for the weight - a small bank of batterys and a high out put alternator as well as sound deadning the car and a front stage that can keep up with the subs = dumping tons of $$ in to a car that you may or may not LOVE.
i dont recomend people to build cars that they dont LOVE... no sence in droping that cash on less than ideal for what you can work with ya kno ?