Ive only dug 9 post holes (175 more to go - 9" auger) with my TC25D so I'm no expert but I don't think hp will help you dig post holes better. (Someone correct me if I am wrong /w3tcompact/icons/smile.gif)I run my tractor barely above idle when I am digging. If I increase the rpms I break more shear bolts when I hit rocks. If I'm just above idle, the auger will bounce and, if the rock is small enough, it will work itself loose and come up. When I do hit rocks, it jostles the tractor a bit but nothing major.
Of the nine holes, all of them are 36" deep except 2. Those 2 holes are about 30" deep. In each case I hits rocks I could not get through. My only recourse would have been to use a sledge hammer and something (digging iron?) to break them up. However, I had no idea how thick the rocks were. In one case, it covered the entire bottom of the hole while in the other case it covered half the bottom of the hole. In both cases I would have 'destroyed' the hole to get the rock out. So, I used 2 bags of concrete in these 2 holes instead of 1.
If you know you're going to hit rocks, you might want to seriously consider a digger with a slip clutch. Also with rocks, you will probably have to get off the tractor to clear rocks out of the hole, I know I did. In such situations, resist the urge to try it with the auger running. Stop the auger, clear the rocks/dirt, get back on the tractor and start the auger.