Sorry a bit late to the discussion HHSA. We have had windows built into several of our #1 Genoas all panel sails. The joke is on a close crossing the skipper can see the competition just before you hit them. There is no visibility in the sector covered by the #1 particularly in the top of the wind range. The window makes it so you don't need send crew too leeward to check for traffic quite as often. From the skippers normal position, if boats on the opposite tack are aft of the leach you may be crossing, but if they are in the window you'd best be on starboard or ducking. The window needs to be quite a bit higher than you think, since it gets the most use when you are at maximum heel angle.
