It really is night and day but putting an amp or two is highly advisable otherwise you will get distortion at higher volume and an amp will also improve the sound quality greatly, well if you install a good amp.
Ideally those bmw need 6 channels at least with the underseat woofers dedicated to their own channels.

I bet that sounds lovely now. I'm gutted that mine came with the basic set up so hasn't even got an amp or tweeters. I'd love to upgrade, but I guess it would mean a full new OEM wiring set up