i lelveld my wl to 90 few days ago. Now i heard that until a certain ilvl destro is performing better thann aflliction over all.
can someone tell when when it is good to change to affli / play affli at some bosses ?
Destro is most likely outperforming Aff and Demo untill 525'ish ilvl.
Some encounters favors the other 2 specs, but I think you are best off playing Destro untill you reach the 525'ish Ilvl with UVOLS and Legendary meta.
The best way to find out is to use Simcraft to sim it for yourself. There are many factors to consider, but the above poster is correct.
If you can get the Legendary Meta Gem, and two RPPM trinkets (LFR), Affliction will edge out the other two specs at an earlier iLevel.
If you can get the Unerring Vision trinket for Demo, that is a massive dps gain, particularly for multi dot fights like Council.