For our models, the best TE is on a surface that tapers back to the smallest squared off TE that leaves enough material to have sufficient strength to stand up to hangar rash.

Elevators that haven't even been tapered at all work ok when the elevator wasn't thick to begin with. If the surface is thick and untapered there will be induced drag as a result. And the drag is wasted drag. There is no return of value for it's cost. Taper the surface to a thin, square TE and you don't have the "surplus" drag. BTW, this surplus drag can be the source of flutter. It's not biased up or down and certainly doesn't uniformly pull the surface in a stable direction, like straight back. It is basically an unknown that will just as easily introduce a flow that actually pulls the surface up or down, and sometimes rapidly and most certainly unstably.