Even an external amp with the same power would sound better than the radio's amp. I have a 45x4 JBL amp even though I don't listen to music as loudly as I did years ago, it's still capable of delivering clean audio as loudly as I care to listen to. The JVC head unit the car came with has a noisy amp. It's too distorted.
The crossovers on seperates does not deliver as much power as what's being fed to them, at least that was the case back when I was in the car stereo business.