So if the aircraft is built with particularly short permanent wings
with facilities toi accept extensions from the start, you have the
choice of an aircraft with higher cruise speed but accelerated
takeoff/landing speeds, or as you mentioned in your posting, an
aircraft with reduced speed on takeoff/landing, and a higher
efficiency, albeit slower cruise. And of course with intermediate
size extensions, you could have something in the middle, like most GA
aircraft have these days- not too fast, not too slow.

At a specific speed those L/D numbers may be right, but I doubt they
hold for the entire range that aircraft operate in. Drag- One of the
reasons I say the wing is one of the largest contributers is because
similararly powered side-by-side and tandem 2 seaters have top speeds
in the same range, but the frontal area of the fuselage is quite
different. Gear fairing adds a couple kts.