I dunno. Booming thermals, high cloudbase -- you
could probably do 1000km in a PW5 in Namibia
if you just took one there.

You could, right.
But reality is narrower: usually no one does.
Last but not least: when coping with normal averages, still possible exceptions are averaged out, by definition.

What could happen in theory is one thing, I see your point. What *usually* happens is what I care most for GliderReview.