Yes it should be across the board because it’s independent of rotation, environment, mechanics, etc.
For example. Say you have a lousy player and do 1500 DPS, while Rocky Superplayer is doing 2500 DPS.
After the buff, the lousy player should be up to 1650, while the good player is up to 2750. Lousy player is still lousy in comparison to good player, but lousy player is contributing more to the group because the boss health hasn’t changed.
If only some abilities were buffed, then it would depend on all sorts of factors.