Hi,
I sail my P19MX mostly ever week, year round. In the winter (as you know we do have winter in Florida), my wife doesn't care to go out if it is below 75. I weigh around 205 lbs. with all my gear, and I don't really have any problems with main and jib to 10-12 knots. After that, I sail under main only. If the rig is set up right, I really don't see any problems at 12 knots with your 300lbs. Remember, the square top will vane, and move the power lower. The sail plan likes to accelerate the boat, with less heel than the original p19 sails. I have found that the rig likes her main sheet "tight". If you start running the traveler out (going to weather of course), you will find the main getting back winded from the jib.
Where I used to "pop" the windward hull up in the puffs with the standard sails, the MX package wants to accelerate the boat. Have fun, it is an awesome package.
Dave