I know there are a few posts on here about people doing this before, and really if you wanted to replace your back two subs i think they only support 6x9s at most. If you go this route don't expect anything super to come out of it. I know you said you only wanted factory stuff, but you would be surprised how little room a 10" takes up, just build the smallest recommended enclosure and mount the amp right on top. And you know that small dip in your trunk as it goes further back, if you go with the 10" or any size it will stay the same size throughout as thats the factory subs hanging down back there. So you will also gain a little bit of room aswell. I know this isn't what you wanted as a response, but depending on how much boom you actually want, I would say just sacrifice a small corner and get a 10" or even a 12".