This is wrong on so many levels, look, relook what the value of the #8 pick is, than evaluate what Evans would be worth if other top WR were only worth a top #20 pick and than resess your trade. But please, don't forget that only Herbert is a somewhat typical BA QB and that Herbert is going higher than Tua and will not be there at #8
Mike Evans would be worth 2 firsts. He is a top 3 WR in the NFL, 26 years old, and under contract for 4 years.
Moving from 14 to 8 is probably worth a first, 3rd, and 1st next year.
Patrick Peterson is worth a 2nd with only 1 year left on his deal.
David Johnson is a wash, and would cost us a pick, or taking on some salary just to get rid of him.
So in terms of picks
We get 3 firsts and a 3rd
We give a 1st and a 2nd
I know it's not that easy to quantify, but I think we come out ahead in this deal. We could always ask for OJ Howard or Vita Vea as well!!