your description is mostly right because like any system there are all kinds of caveats, if thens, buts, and so forth.
at a high level, that is basically it. 'chop is facing this same dilemma in his seosr mega build. he has dual 15's for this mains and is concerned that it might not be enough destruction. td15m's in his case. iirc the current plan will be to go with the dual 15's and see if that is enough and if for some reason it is not, double up. his system is full active though, so adding in a second set of drivers is easy. with passive mains, we run into one of those "buts".
once you have the subs, fire it all up and see what you think. if you don't know what an 800 horsepower car feels like to drive, no sense in wondering if it will be enough. :-)