Arjan, though not being the expert on trimming, I remember me a case in which I had a sail which did't fit with the mast.

That looks a bit on your situation. Sailing with two men in 20 kts, should be no problem. Also not in half-wind (the official English is "reaching", but here on this forum they like to speak of jib-reaching or beam-reaching).

It seems to me that your sail cannot loose all the wind in it, which results in an (inefficient) force forwarddown. In fact it's a twist-problem.

So, take your sail one time on the ground with you holding the cunningham hole and your mate holding the top of the sail and stretch out (but not so far that the sail is curling upwards).
Then you take a rope and make a straight line also from the top to the cunningham. You can now see the pre-cutted luffcurve of the sail and also measure how much it is on the deepest point.

If this curve is too deep for your mast, you have a problem.

So, in that case I would certainly give the mast much more pre-bent with the spreaders. (For your cat I don't know the appropriate figures for the pre-bent and the prescribed luff bent in the sail.

Last but not least I would fix the mast-rotator to zero.