Tornado Shot gem links

I've looked at a ton of Ranger builds and can't figure out why NONE of the builds I've seen support Tornado Shot with Volley Support, which fires 2 additional projectiles. Almost every Tornado Shot gem link includes Greater Multiple Projectile Support and usually Mirage Archer Support. I've read how additional projectiles will be a large dps increase, but I've never seen Volley included in others' suggested gem link list. Volley seems to work (at the cost of 40% more mana). What am I missing about Volley that seems to make it only worth pairing with Barrage on a secondary damage chain?

Because we have wider spread with GMP so that's means that the clear speed is faster.And also for bosses because of the angle i think GMP still better cause more projectiles hit's the boss if aimed correctly behind the boss.Check tornado shot aiming tutorial in youtube if u don't know about that.

why not GMP _and_ Volley for 2 additional projectiles?