It seems to work for both lower and higher AOA's. There's some interesting discussions in the archives of the boatdesign.net forums on just why this might be so. The gist of the arguments seems to come down to the headsial's finer entry or leading edge. The mast is the LE of an una. The blunt LE of the mast allows the stagnation point to slip over to leeward at low AOA more than the fine, sharp LE of a headsail. So then the mainsail is actually operating at a higher AOA and making much more power. Seems to make sense.

I don't argue about the performance of the A cat; I'm sure it is all so many people say it is.

My sole experience with unas was with my first cat, a Hobie 14. I added a jib after a while and found it did point noticeably higher.

Jimbo