A 1:2 needs more muscle to pull up/down, especially if the kite partially fills. However, Kathleen seems to manage OK on Gill's Blade which has a 1:2 spi halyard. But like you said more rope alongside the mast does affect the airflow and in Gills case contradicts the internal downhaul. I wonder how much performance is lost in real terms with having the spi halyard up the mast compared to the windage factor with a 1:2 system on the forestay? not even measurable I would imagine but more psychological.