I don't disagree, but in your list are parameters which
alone will give a better MPG by flying
slower.
If you have a lower wing loading / a lower stall speed (a requirement on microlights and other "ultralight" stuff) then you end up with a plane whose optimal cruise speed is lower, and that alone will produce a better MPG.
The Long-EZ is a tandem, which will easily be hugely more efficient. It also does away with the elevator drag causes by the elevator pushing
down in most traditional designs. Canards are more efficient due to this.
The rest, like wing root fairings, are little details which all help. My TB20 is covered in pop rivets and it's obviously cheaper to do that than flush rivets, and my guess is that Socata didn't think it is worth spending the extra money to get maybe a few extra kt.
It is a known fact that the retractable footsteps on the TB20GT are worth an extra ~2kt; Socata apparently did that to preserve the POH performance figures despite the taller roof, because any change to the POH creates a lot more certification work
Aerials can make a big difference too. If you have 2 x VHF, ADF, DME, XPD, that is probably 3kt down the drain.