Most factory radios can go all the way up without distortion. This is so the amps/speakers will not get blown by user negligence.
The radios are different. The JBL ones do not have amplifiers in them as the amps are mounted in the trunk. The non-JBL stereo's (no amps mounted in the trunk) better have an amp inside them, or they wouldnt work at all.
You may be able to get a bit more out of the system, but the trouble you would go through to get an extra 10 watts probably wouldnt be worth it. The speakers and amps are probably spec'ed so that they sound clear at max volume, but start distorting slightly above max amp output.