Hanging IKEA Kungsfors on drywall
I would say that the snap toggles are a lousy way to hang a shelving unit like that if only mounting into drywall by itself. If you had any hope of using the shelving unit any where near its load capacity the drywall only solution is a total non-starter. 

I see two possible solutions to this situation:


Install a horizontal strip of wood across the studs aligned with the mount point on each bracket. These are secured with screws into the studs. Other screws then secure the shelf rails to the wood strips.
Apply a plywood backer against the wall behind where the shelf will be installed. The plywood is screwed to the studs using numerous screws. The shelf rails are then mounted onto the plywood using suitable wood screws.


In both cases the wood/plywood can be painted to match the wall color so they do not stand out. Of course in the case of the plywood it could be finished in a way to look like a back for the shelves if it's size matched the full width and height of the shelf unit.