That is an interesting channel, one of his other vids does show a bit of gains for diffuser only, but way more for a flat underbody. Not too sure on viability of going flat from front to back though, center is pretty flat anyway, maybe a section over the diff/axle/suspension pocket to join up with the diffuser. would want to direct some air flow over the diff though, that gets pretty hot already. And sort out a more minimal exhaust option at the same time. Always more to do!
Got it stuck together with sikaflex today, and filled the internal gaps along the 'rolled' bit. Few gaps along the bumper fitment to sort out still, plus mounting positions and hardware etc, then paint and final fitment (maybe next weekend).