Do the factories offering polyurethane paint as an option to gel coat
still use a thin layer gel coat? How much does that method extend the
life of the gel coat?

Yes, it is necessary to use gel coat or an in-mold primer gel coat. As
Hank mentioned it is not practical to laminate directly on the mold
surface. The laminate will be full of pinholes and require extensive
post-mold priming and sanding before a finish coating can be applied.
Now here's a catch 22 - It is not feasible to apply a "thin" layer of
polyester gel coat. The normally recommended wet film thickness ranges
from a low of 14 mils (mil =.001") to 18+ mils. With anything thinner
than 14 mils the proportional amount of styrene lost through
evaporation becomes so high that there is not enough styrene left for
complete crosslinking. Bottomline - overly thin gel coat will be
permanently under-cured regardless of catalyst level or curing
temperature.

A linear urethane topcoat will extend the life of a well applied gel
coat substrate condsiderably.

Bob