Andrew, I understand you're a bit nervous.

Can you provide test (UV, mechanic, abrasion...) to confirm what you write ?
Low tech people, as I am, need some serious stuff to believe you now.
Sales representative one hand argumentation are not serious enough.

Why 1,5 mil the limit. 1 mil or 0,75 could be ok ?
More seriously 1,5 mil. are lighter than 3 mil. sails. That is a performance bonus for the boat.
Weight in the high of the boat is very important.
You're not agree, with that technical matter ?

Plastic weight is like gelcoat weight, that is dead weight.
You're not agree, with that technical matter ?

150 kg boat are cheaper than 180 kg because they use less material.
But they are no more F18.

Most of the F18 fleet is gel coated and with 3 mil. sails.
And the interest is to have the more similar boats on the starting line.

Some do not need faster boat to be in the top ten ;-)
They're simply better sailors.