As a previous J/30 owner for several years, I ran into the same issues with sail cloth. Since there was some one design to be had in Western Long Island Sound, I ended up with two sets of sails. I believe that the kevlar mains I had performed slightly better than the dacron mains. On of the benefits of the hi-tech sails is maintaining better shape over time. The last year I had the boat, I won our Wednesday night PHRF series with a 4 year old Kevlar main. One the other hand, I won BIRW in PHRF using a Dacron main with about 4 regattas on it.
Overall, if I still had my boat, I would prefer to have the flexiblity to choose mainsail material, but would not fight hard for it.
On a different note, the 0.5oz chute is essential for PHRF racing in light air venues. Someone said just unload all the crap off the boat. That doesn't really fix how the sail flies. In the light stuff the 0.75 may fill, but still sags while the 0.5 flies alot higher with better shape. On LIS you rate the same in PHRF with no sail limitations and a 1260lb weight limit as you rate in one-design with 1400lb weight limit, so it was easy for me to give up the weight for the spinnaker and other sails (especially for genoas before the rule change).
[This message has been edited by Ed Austin (edited 06-26-2008).]