Given the OP's post I don't see a heavy side load as likely.
The self feeding auger (and self feeding spade bits) have very aggressive feed rates and therefore need a low speed/high torque gear. With my old XRP you could force the drill but there's no way to do that on the new 20V max. In third gear the electronics shut the drill down even with a 1" self feeding auger into a new 2x4.
I'm not answering the question if they should, just pointing out they can't. The OP's expectation to be able to use largish bits on high speed is not realistic with the newer drills. Also, while you could be aggressive with the old XRP's you may not have gained much anyway..
One finial point on the OP's question. Knowing max torque won't help if you don't want to use the low gear.
That's the point. The DeWALT's (both 18v and 20v MAX) are more than able to do the 2 9/16th inch bits. If you are doing a lot of holes, then obviously you will be happier running a cord, but the point is that the cordless versions will in fact do the application quite well. All that was to say that it sounds odd to me that they are having trouble with such a smaller application unless they are under VERY heavy side load or bind.
