Wouter,

I regret to prove you wrong but I do repeat my statement.

I had the opportunity to weight up an "Extra-Light" Oxosails mainsail (Cuben Fiber/special Oxosails carbon battens)at an International A class event:
it scaled up at 2,1 Kg.
Oxosails has a remarkable track record for A class mainsails. Have a look at their official website to verify:
http://www.oxosails.com/prod01.htm#a%20class.
The owner, Manuel Vaccari, is a well known and respected A class guru and an excellent A class helmsman too. He is, let's say, a "cutting corners'" specialist.
If you read all the features of his latest A class design you'll find many interesting details and also the confirmation of my statement:
he says that his latest A class main weights from 1,9 to 2,3 kg.

The true is, as Bill underlines, that most sailmakers build you sails on the heavy side only to be safe. However it goes without saying that Oxosails main are not built to last: they actually last 1/3 of BIMARE mains and also BIMARE mains win important A class event (the latest being the 2005 Italian Open).

The choice is upon you