Yeah...I've used that Optoma when I needed a longer Throw Projector. Part of your problem is the older A-Lens's optics....the rest is owed to the PJ's LBX Mode's minimal ability to process the "amount of Stretch" needed. The HD28DSE's LBX mode was/is intended for use with Letterbox sources (non-16:9) to stretch to fill a 16:9 area at full resolution.*
The Panamorph Lens is really intended to change a 16:9 source into a 2.35:1 format. *The HD28DSE also has a "Superwide" Mode to fill a 16:9 area, but that crops the ends of the 2.35:1 image....)
Projectors that have more intensive Stretch Modes to stretch 16:9 Content do so to counter the effects of A-lenses.
But there are physical / optical limitations inherent in certain "older" A-Lens, and those limitations are tied in with Throw Distances and Image sizes.
Under "normal" circumstances, (ie: smaller screen width) you would not see enough pin-cushion overspill to matter...and really, it's more a case where one doesn't produce noticeable image distortion at the edges. Again...at a smaller size, nothing to worry about.
The newest Generation of A-Lens optics have virtually nada pin cushion issues. But they also come in at Used Car cost. And try to buy one with a Motorized Sled?
Anyway...your going with a epic sized screen, so you solutions must also be geared toward compensation, not necessarily optimization. To do what you need to do, ya gotta warp the Screen's shape....quite a bit. And it seems you have a good plan, so get'ter dun!
.......and save the Eggs for breakfast.