Square tops: 20 and 30 years ago, they made skiffs and cats with comp tips or flexible masts, the idea being that in higher winds, the top of the sail would 'twist off'. Today, if you load up a mast like that with a square top, the 'twisting' that occurs will result in a leech that is not tight, and you lose performance.
All the modern high performance boats (29'er, 49'e, Cirrus, WildCat etc) have solid stick masts. They were designed to carry a skinnier mainsail, with a square head, plus a spin. the thinking changed, as did the design. We're now into more of a 'wing' sail, so that's why the 'old' and the 'new' don't always mix well.
My biceps put on an extra inch as I would have to crank down on the mainsheet to tighten up the leech.... and we loosened the rotator to allow more flex in the mast.
Time to buy a Phantom?
http://vimeo.com/83829320