I would suggest a single 15" in a ported box thats as big as you can get away with. same for the port, as much port area as possible. I personally would tune between 32-35hz. If you like loud, I would do the 35hz.
The reasoning being, this setup will be most efficient... get you the most from the least money spend. airspace is free, a second sub and bigger amp is not.
As far as keeping the factory mids/highs I think your on point. If its not an all out sq car and you are already happy with them. why not. My daughters ride is still sporting the factory paper speakers and tweets with a ported 15". I'm amazed everytime I jam out in it. Granted it has an Alpine headunit that allows quite a bit of tuning.
Lastly SSA sub's are F'ing badass!!!