I understand about the importance of getting centre of gravity right, and what can go wrong if everyone bunches at one end of the airframe, but... if 25% of the rows are going to be blocked off, leaving the pax to fill the remaining 75% of seats, then on a load factor of about 70% this means all the seats are essentially taken with pax crammed together.
The obvious conclusion is:
Why would anyone purchase 'priority boarding' from a LCC when they know it will make little different to the quality of the ride they get for 3 hours anyway ?