For a constant sail area, square tops have a couple things going for them: they have more area aloft for better light-air performance. In a breeze and above, they benefit from their closer-to-eliptical shape and higher effective aspect ratio, both of which reduce drag.

The place I expect square tops to be clearly inferior are in survival conditions, where the wind is so strong that travelling down is not enough, and you must ease the mainsheet to depower. When things get that ugly, the pinhead would definitely help... but those are not typical race conditions.

In double-trapped-and-depowering conditions the square top has a higher effective aspect ratio, and the pinhead has lower center of effort. I would not expect a big difference in these conditions, but I'd expect the higher aspect (lower drag) square top sail to point higher, which can be a tactical advantage, and compensates somewhat for the reduced power.

Kaos points out that the 5.8 can be hard to depower. I'm not speaking from experience, but the 5.8 system (with clew track on the main) should work fine until the clew track car is fully aft. From that point on (in increasing wind), you're forced to over-flatten the foot of the sail, but should be able to depower effectively until the main traveler is fully eased. After that, things get ugly even with a boom, but especially without a boom, when the Kaos effect kicks in, and easing the main sheet powers up the sail.
In other words, the lack of boom should only effect you in significantly over powered reaching conditions.

As for sailmakers, I'd give Skip Elliot a call. He has lots of Nacra experience over many years, and made a boomless 5.8 sail for a friend of mine, made an oversized square top retrofit for another friend (on a Mystere 6.0). Both my boomless and square-top friends are quite happy with their sails, AFAIK.

--Glenn