No experience with an extended boom, but using trig to calculate the center of effort of the main shows it moving aftward by about 8 inches (from 55" to 63" from the mast). This will increase weather helm by some amount, all else remaing equal. How much , I don't know.
In addition, although the center of effort will remain the same height above the deck, the increased area will produce more force, thus causing increased heeling, which will further increase the weather helm.
My own personal experience is that, due to her stubby shoal-draft keel, the 323 likes to sail "on her feet". I usually go the other direction, putting in the first reef at 15 kts, and the second at 20 kts.