Very good idea money wise... the only problem and I guess the reason why they are going to trial it is - ON TIME DEPARTURE ...
One of the reasons why Ryanair have such a good record for on-time flights is the fact that all the passengers are at the gate ready to board long before the flight has even landed. I think even if you just sold exit rows for £10 the people who will buy them will just spend more time in the cafe and shops to avoid the scrum, I guess thats why they are adding priority boarding but if it was me i would still rather turn up at the gate after the scrum and take my seat...
Why not just go the whole hog and have a Ryanair first class maybe named "Golden Harp" seating 2-2 stick a gold head rest cover on the first 5-10 rows free 2pc 30kg hold baggage free drinks/snacks and paper maybe even a portable IFE unit and charge an extra £100 on ticket price, if the seats arnt all taken the people who have payed for prioraty boarding only could get a free upgrade at the gate this may make more people buy prioraty boarding on the off chance they my get a free upgrade...
This will also attract more busines travelers...