I feed Raw but I have Orijen in the house for occasions I need it. I think It is an excellent food, and the best you can get from a dry food. (I don't think it is as good as Ziwi but I cannot aford that
I have not tried Acana but I don't think it is possible for it to be "too rich". The only thing that would make a food less rich is by adding in more non nutritional fillers and less quality meat. Remember that a dog's digestive system is designed to eat only meat not corn, potatoes etc. So a dog can never really have a "too high" or "too rich" concentration of meat in its food. I think if a dog is gaining weight on a food, just reduce quantity a little. What we have to remeber is that the instructions on ammounts to feed on the back of the packets are often over estimated as they want you to go through the bag and repurchase as regularly as possible. When I put the wighed ammount of James Wellbeloved food out for Feodore when he was a puppy (recomended for his weight, and left out 24/7) it took him three days to eat it!
With any dried food I would say- start off with the guide ammount from the packet then adjust by looking at your dogs shape and judging if they need more or less, because every dog is different, just like people.