Nothing wrong, just not enough umph. My already owned Alpine system (Head unit, subwoofers, 6x9's, and amplifier) kicks out a lot more juice than the stock Bose system. I would have to say though, the bose system is not too shabby for a stock system.
Yeah it probably would, i would read your owners manual, however if it does i would go to a car audio place have them install it and they will honor their work for a year anyway. (Atleast my shop does),
To be more specific as to the advantages of a new head unit, standard systems do not draw enough energy to play to the best of the cd or speaker quality. A new head unit will dramatically increase the clarity and possible loudness of your system. The downsides are the cost as well as the inconvenience of having a weird looking stereo system, but many systems are a lot more attractive then standard.
Blackeldo if you are fantaical about car audio you need new speakers (the bose are pretty good though) (try fosgate audiophile or focal speakers) and an aftermarket amp going just to your speakers.
Also be concerned about how a new head unit will look in the dash. Unlike the 95- cars, the 96+ cars had a radio shaped just like the center console -- meaning that it's not a standard DIN or double DIN, etc. Do they make adapter plates now, to fit in a Seville/Eldorado's radio space?
Unless the dealer can prove that the aftermarket product caused the failure of whatever when you take it in for service then they cannot void the warranty. For example, you changer your rims and your tranny starts slipping... If they even bother, I'd suggest to find a new dealer and badmouth the one that refuses as much as possible. Walk up to people on the street and tell them about it. Email random people from your address book, write your congressman about it. Up to you.