you should get the full range the speakers and source are capable of.. you're not going to improve bass response though if that's what you're after. you need at least a small powered sub for that.. especially on factory systems
the 512i infinity coax speakers I put in are only temporary until my Alpine TypeR mid's get here. I have a set of cheap tweets coming to replace the stock tweets too. At least until I can find a set of alpine typeR tweets. at least mounting depth won't be an issue in my doors.
there are advantages to using coaxials. one benefit is that the tweeter and mid are so close together. you get a near perfect point source, avoiding potential time alignment problems that can degrade the realism of your music.
but coaxials can't usually make strong midbass like components can. it's a trade off.
Coaxials generally don't have a good crossover either, and poor designs can cause the tweeter's location to interfere with the mid. Well designed coaxials are nice, such as the Eclipse point source coaxials and the CDT braxials (those are more or less components, though). Component tweeters can be made to be even closer to proper phasing if installed correctly, it's just that most people usually put the mids in stock locations and the tweeters up very high in the a pillars or upper door.