the backpack will only throw off your balance making it harder to concentrate on where you are in the air. it will not help at all.
only weighing 120 does matter IMHO. i weigh more than that but i'm still a lightweight, and we skinny fucks have to really find the sweet spot in the tail as well as having to work harder to keep speed. 160-170 is about perfect for shred gymnasticalisms
you might try messing around with the mounting position of your back binding if you think you aren't getting good pop every time.