lots of attics and even new construction space is a premium.
one 10" duct with 6" of insulation won't fit in a lot of spaces.

having an attic with lots of space is an exception..most are
tight cramped spaces.

I think everyone agrees that having more insulation on ducts in attics
and crawlspaces is a good idea. that we put ducts & equipment in those
spaces is just crazy.
unfortunately the hvac industry has not found a way to increase R-value that works
in multiple applications.

it takes a lot to bring about industry changes.
good examples are the 2005 move from 10 seer a/c to 13 seer a/c as minimum
efficiency. another is pending May 2013 elimination of 80% gas furnaces
in northern climates.
efficiency is improved in both examples, but it took a long time
for the industry to mandate these changes.
ductwork efficiency has gone from R-2 to R-4 to R-6 & now R-8 in most areas.

in my area installers complain about the difficulty of attaching R-8
ducts to plenum & supply boxes. to be able to attach 6" of insulation to
start collars & supply boxes would require modification of these pieces.

is this for your house..or your sister's house in florida?

best of luck.