It's interesting to see a few people feeding their non working dog on "working" food. I asked a couple of people - two pet shops, folk at the recent dog show, our vet and even the manufacturer, Burns - and they all said the same thing: working food is intended for dogs that are highly active for 5+ hours per day.
Posie is very active and can keep the effort up for hours at a time but because this isn't the norm I was advised to stick to the regular food.
We recently switched her from a juvinile dry food to a proper adult one and the change in her behaviour was quite marked. To explain, I had misread the label as "small adult" when it actually said "adult toy breed" so she was getting far too much protein. Now she is much calmer (apart from some recent yapping).
As I understand it, the issue with working food is the very high protein levels.
We still subsidise her kibble with fresh meat so her protein intake is still higher than may be needed, but she has shown no sign of putting extra weight on despite being given portions that are supposed to be for a dog 2-3kg larger than her.
She is a very lean and fit dog, and resembles a body builder in her muscle tome, rather than the sleek, almost skinny tone often associated with Working Cockers