So an airplane that is designed for a utility class rating at
1600 pounds should be able to withstand about 1850 pounds if
the G is limited to the normal category limits (4.4/3.8 x 1600)

Ron, I have the greatest respect for your posts, but this is not quite
right. When calculating bending moments for sizing the wing spar root,
the weight of the wing should not be included. The wing, in essence,
is self supporting and only the fuselage (and everything in it) is
being lifted by the wing.

So for a 1600 pound gross airplane, let's say the wing weighs 200 lbs.
Moving from utility class to normal category yields the following:
1600 - 200 = 1400 lbs
1400 x 4.4/3.8 = 1621 lbs
1621 + 200 = 1821 lbs new gross weight (not 1850 lbs)

BUT WAIT! If the aircraft has fuel in the wings rather than a fuselage
tank, the weight of the fuel is also not included when calculating spar
bending moments (because the fuel is in the self-supporting wing). If
we assume the fuel also weighs 200 lbs, our calculations for a 1600 lb
airplane might look like this:

1600 - 200 - 200 = 1200 lbs
1200 x 4.4/3.8 = 1389.5 lbs
1389 + 400 = 1789.5 lbs new gross weight

Now we are seeing significant difference from the suggested 1850 lbs.
Even these calculations are an over-simplifcation. My point is, one
has to be careful about making engineering assessments without
reviewing the original stress report.