I have vertical battens in the main and mizzen. As long as the boom is at 90d, furling hasn’t been a problem.  Sail shape is significantly better than without them.  I don’t know how much that effects boat speed, but it probably helps some.

My battens are 20mm wide and 6-7 mm thick.  I can imagine that any thicker might be a problem going into the slot.

I am going to ask my sail loft about adding luff foam to improve sail shape when partially furled.  I don’t know if the battens will interfere with that.