So for my question; is the Disc Trucker at least 62.5% better than the Long Haul Trucker for my purpose? I've never had disc brakes and I've never really felt the need to have them.
Personally, I'd say no. I have a
non-disc LHT from 2009 (purchased as a frameset), set up with linear-pull brakes and appropriate levers, and I've always felt like the braking is excellent. I've ridden through the Cascades and Appalachians fully loaded and have never had even the slightest sense that I needed better brakes.
That said, if you read through the forums you will find others who feel strongly that discs are much better. And, if you ever think you might wish you had discs, then the upfront cost now is certainly cheaper than changing your mind down the road.