If you drill the hole bigger than the recommended size, TapCons will NOT hold. DAMHIKT
As for the impact driver snapping them easier...yes, it does produce A LOT more torque, but seems (IMO) to exert less force on the screw and it goes in smoothly. I snapped more heads with my drill than I did with the impact. Doesn't make sense, but that's how it worked for me. I wonder if the impacting (hammer action) has anything to do with it.
Be well,
Doc