I opted to use a similar throat model in the LaScala as you used in the Khorn. That is, calculating throat area (S1) at the "splitter" after the horn doubles into two paths. Then entering the slot area as (Atc) area of the throat chamber. I also lengthened the last horn segment and shortened the middle segment slightly maintaining the overall length. This was to prevent the "Cir > 1" warning. The help file in "Hornresp" states that results are invalid if this message appears.
I have modeled a lot of horns in "Hornresp". It takes fairly big percent changes in a model to make a significant difference in the SPL output. After all, a 6b change requires the acoustic pressure to double.