I don't think a sub is too much.. a sub is the way to go for getting the low end in there... not just for that rattle noise people want for listening to rap.
i have an alpine HU and infinity reference speakers (kick panels, and tweets), as well as the rears that came with my car.... i have a custom made box the fits well into my trunk not taking up a whole lot of space that holds my infinity reference 1230 sub (12 inch) and amp.
when i disconnect or turn off my sub, i notice a big loss in low end. turning it back on..its amazing on how big of a different there is.
no, a sub is not too much, but it can weigh a lot... my sub, amp, and box weigh wayyyy to freaking much...but whatever.. my car is slow anyways... m42 engine....