My question would be, is all this worth it for 15L vs 19.5L of fuel for a theoretical extra 100km of range?

I can consistently get 300km out of my standard tank on an "adventure ride". With a little Rotopax on the back, I've got the same range as if I'd ruined the look of my bike by fitting a Gillimoto.

And I can take the rack & Rotopax off put it on another bike..