A recipe that calls for 8 oz of chicken would be raw chicken, before it is cooked. If the recipe wants you to use cooked chicken it will say so. Like a casserole or salad will say 1 cup cut up cooked chicken or 8 oz of cooked chicken.
With W W you can estimate 4 oz of cooked chicken to be 4 pts. 1 point per ounce. Yes chicken does shrink once it is cooked so it is important that you weight it once it is cooked.
I bought some chicken breasts on sale that were boneless and skiness and quite large. I cut them into what I estimated would be a 4 oz serving once it was cooked. I then packaged them to freeze. I took the left over odd pieces and sizes and cooked them in water for 20 minutes on low once they started to boil. (maybe less time if the pieces are small) When the left over pieces are cooked cut the chop up the chicken or shred them. Package in 1 cup or more zip bags and label and put in the freezer for a future casserole, enchillas, tacos, salad or sandwich. A great way to get supper on the table quicked, especially on these hot nights.