I can guarantee you the stock system does not deliver 20 watts per channel RMS. I would be genuinely surprised if it could produce more than 10W RMS per channel.Stock radio is pretty bad. Im thinking 20 watts per channel. If you grab a amp running between 250 and 400 watts. (between 50 and 90 rms) Your stock speakers will handle it just fine.
I generally only refer to speakers and amplifiers as RMS ratings. Peak ratings are useless.
The speakers in the stock Cruze are chosen for the power capability of the head unit. Adding a more powerful amplifier to the stock speakers will not do any good.