I assume you are the only one with that boat/sail there?

If you loosen the two lower battens (a lot), and then apply mainsheet+downhaul, what happens?

Sailing without any twist at all is usually slow in my experience. But your main should not make an S-twist at the lower two battens anyway. Is the sail specially designed to tweak the class rules, or is it a well known design in the class? What Bill Roberts already has written, is usually the reason for S-bend. Stiffer battens might help, but might ruin the shape otherwise.

Is your mast set up as per the class 'norm' with pre-bend (diamond tension) and spreader rake ?

It would be easier to come up with teories if you could supply pictures of the sail taken from the side and from below (while sailing or while sheeted in+downhaul as you do to recreate the problem)