I've sheared lots of the turbo ultras
, although you may well be OK with a 6x110mm. They are generally fine in softwood, it is oak that snaps them even when pre drilled.
The slotted stainless are good and I have never had one shear, but they are a pain to use compared to posidrive; I bought a load of Spax stainless, I think from SF, and they have been great, no shearing and drive well. SF also had some square drive stainless and they were also good.
But if the battens are behind the cladding they will not get any weather and at a pinch you would probably be OK with a quality, Spax or Reiser, steel screw.