In my thinking you need the load out auger a size bigger than what ever the horizontal discharge auger is under the bin floor.  When you start taking that grain up at a 30 degree angle or more that auger just does not keep up to a full speed open trap on a horizontal auger.

So a 6 in. horizontal discharge needs a 8 in. incline.  A 8 inch horiz. needs a 10 inch incline, ect.