according to a couple of body guys that i have spoken to recently ... they don't agree with you. and they have done it both ways ... with just the w+n parts and with the full floor pieces. you should touch base with Tony Dreamer, i think he also has first hand experience with this discussion.
I am of the opinion that the front and rear W&N panels overlap each other. When using them both, no need to fill any space in between. Au contraire, you have to trim some excess material.
Fixed the right side already like that, from subframe mount to firewall with W&N panels. Now doing the left side.
the answer that maybe was given by the body guys was perhaps an answer to how they undedrstood the question; The W&N , nor the polish floor pieces don't extend all the way to the back (parcel shelf). Stupidly enough, most E9's rust at the subframe mounting bushing and that most rear bit is not covered by the W&N , nor the polish panels. Your body guys may have had that in mind when they commented.