[Christ]

So if you're going to extend the air dam, shouldn't you relocate the belly pan to the new lower edge, to keep the flow under the car attached as well (instead of creating a micro-wake behind the new valance), or is this insignificant?

Intuitively, based on what I know about fluid dynamics, you'd want to lower the leading edge of the belly pan from where it is now, and connect it to the low point of the new front air dam, leading the angle from the front to about 10* if possible before meeting up with the rest of the body's under side.