I really think the dacron mainrestriction is fine. I race in a very competitive PHRF fleet of J-29's, S-2 9.1's, Olson 911 and 30, Santana 30/30 as well as a few other J-30's. Most of the other boats have high tech sails all around, and I see no noticeable performance difference between us on that basis. Age of sails and boat weight vis a vis conditions are the key points (along w/ boat handling and tactics of course).
High tech jibs are much more useful because headsails have fewer possible adjustments to shape than mains-a jib that holds a proper foil through a broad range of wind conditions is very important. The main has multiple adjustments to tweak the shape in conjunction with pressure, so having low stretch material is far less critical.
I would encourage you to update headsails before worrying about a high tech main. And I know that I will do whatever I have to to keep in one design configuration-nothing is better than sailing at J-30 OD events!