actually most current spinnaker halyard setups have a fixed point in the middle of the mast profile at the mast top. from here a line is connected to the halyard block which than is also running on a rope [in earlier times this was a metal half arc ring] to allow the block to move to both sides.

so basically as the main force attach point at the mast top is fixed and not rotating there is still a tendency that every spinnaker tries to de-rotate the mast. how much this force is noticeable all depends on the other forces like those from the mainsail trying to rotate the mast and mainly depending on the distance between the spi halyards block running on the rope or ring and that top attachment point. so more distance you have between them, so less the tendency of de-rotating becomes visible.

In lack of a small spi yet I am using the large F18 spis on a 4.9 super wing mast. to hoist those long luff spis my spiboom is very low and still the spi halyard block is very close to the end of the mast [I am aware size of the spi and position of blocks do not fulfil current F16 class rules]. with such a close distance between the ring rope and the top block (I think it's less than half a metre) the tendency for de-rotating is extremely noticeable and I constantly have to use the mast-out-rotator to keep the mast rotated. while this surely helps to protect your mast that even a strong gust cannot easily de-rotate your mast, its tricky when jibe as you shouldn't forget to detach and reattach this rotation device!

I noticed a similar tendency on the A-cat already where the masts (and specially the marstroems I was using) are much more soft sideways than the stiff super wing mast.

Regarding the safety of the mast, feel very safe if you run a 17 m² spi single-handed on the super wing mast. you must do something terrible wrong (like opening the mainsail when burying bows etc...) to kill this mast. sailing double handed the forces on the mast (we talk about dynamic loads) generated by the spi become much higher and that is where in strong wind I start worrying about the durability of high aspects 21m² F18 spi attached close to the mast top... but as long as my mast can deal with those forces those following the F16 specifications should feel really safe with their super wing section!