Each character doing 10% less dmg would be your total group damage going down by 10%.
Reducing the damage by 50% would be like 2 1/2 boxing. Your 5 10% nerfed damage shamans will do an ASSLOAD more dmg than 2.5 shamans.

Example: 5 shamans all 5 do 1000dmg.

5x1000 = 5,000dmg

10% of 1000 is 100 so now all do 900dmg

5x 900 = 4500dmg

4500dmg is exactly 10% less than 5,000dmg not 50%.

5x10% of the total group damage = 50% of 1 shaman, so it would be like 4.5 boxing instead of 5 boxing in terms of damage. If your damage hit 2500dmg then yes you got a 50% nerf but it isn't that way. Math ftw.