I don't know what the factory power ratings are, but remember that the factory resistance ratings on the speakers are abnormal too. Typically, just installing aftermarket speakers that are rated at 4ohms will give you better audio with the factory head unit even, simply because the speakers have a lower resistance, so the factory unit can power them more efficiently. Knowing the RMS for the factory installed system isn't all that useful, because the ratings are at a different load level than what aftermarket speakers have, which means that the power output will be different when you plug in different speakers. If you look at ratings on aftermarket amplifiers, you'll see (at least on decent quality and better anyway) multiple RMS ratings at different ohm levels, and the advertised peak rating is at a specific ohm level as well. The low end super cheap stuff might not provide the same ratings at the same ohm levels, or even provide you with info about what ohm load they expect.