No downside that I'm aware of. GT designs them to be tuned from the front, back, or both, and I've successfully tuned them that way for years. As far as "why" it works, I'm not completely sure of the physics involved, but you can certainly tell the effect on a bare shaft as you add weight to the back: it increases the dynamic spine. If you imagine a very heavy weight attached to the front of the arrow, the arrow has to push it from behind which will cause more flex. If you imagine a very heavy weight attached to the back of the arrow, the weight is holding the arrow back, which straightens the arrow out.
Curious as to how you got the 17 grains. I thought the standard GT insert weights were 20 grains?