BRU is a bit of a mess generally - there's just nowhere for more than one boarding queue to go.
At BRU there are always two boarding queues. There is one for priority and one for general boarding, well separated. The new procedure complicates it because now the priority boarding is in different steps: first CE/gold, then silver, then bronze. They make the boarding procedure announcement very clearly but people do not always comply. They are checking if people have the right priority for boarding and are sending people back who do not have the right priority. I have seen a bronze member who had to go back when only CE/Gold were boarding.
In my opinion the new boarding procedure does not speed up boarding at BRU. It results in CE/Gold to board first though.
The new limits for the hand baggage dimensions are not enforced. People board with larger items, which is normal since it is a very short flight and people not connection at LHR try to avoid checking bags.
Full flights can result in hand baggage storage delays which the agents at BRU try to anticipate by starting boarding early. In general I would say that the agents acting for BA at BRU are doing a great job!