The lower GPM of the pump will only reduce the speed of the auger as J_J points out. Torque is dependent on pressure, not GPM. You will not hurt your pump. You don't need a lot of speed when digging anyway. The only downside besides a slower dig time is that you very well may not be able to "clean off" your auger bit by "spinning the dirt off" (not enough RPMs). Typically you pull the auger up and spin the dirt off and drop down again. You may end up clearing it off with a shovel if you go with the larger model. Rev the engine up when you try to clear the auger bit and see how it goes. If your soil is dry and/or has rocks, you may be just fine. Wet clay will stick. You can always stutter the FEL when dropping to shake dirt off too. You can help matters by doing your fencing in drier months. I do mine in the wetter months because that is when I have time for fences. If you have not bought the larger auger yet, I would go with the 5-11 GPM to allow you to spin clean the auger. Good luck, You will like the hyd auger