The short answer is yes, Pentex fibers stretch 30-40% less than Polyester. This results in a stiffer sail which is generally more responsive and better in the case of a square top beach cat main. I ordered my last set with PE15 cloth, details in this data sheet on page 19: http://www.sacloma.com/DP2017.pdf
I don't know of anyone using fibers less stiff then Pentex in a racing beach cat sail.
Your other choice generally is PE10 (pretty light IME), or the Apen 06 3mil which is pretty standard on most beach cat sails. Per my sailmaker: (Relative to Apen 06) PE10 has extra 30% more fibers in it, the PE15 has 2 times more on warp direction, so elongates much less compared to the Apen 06 3mil.
Cloth weights below:
Apen 06 OD 3mil 4.5 oz/sm
For the F18 I think the weight delta between the Apen 06 OD 3mil and the PE15 is roughly 11.9oz, its probably a little bigger with reinforcement patches etc. so about a 1lb. I decided it was worth it for the increased fiber content and reduced stretch, all of which generally translates to increased racing life of the sails. That being said, in general the longest lasting mains I've seen have been built with the Apen 06 fabrics (Contender Sailcloth) as they have a heavier duty mylar (1.5mil or or 3mil) than the Dimension Polyant laminates (3/4 mil), and mylar is what takes most of the abuse as it breaks down in the sun.
This is fun reading as well if you have a minute, mostly applicable but different application for sail cloth: https://www.willswing.com/hang-gliders/hang-glider-sailcloth-information/