5/3 the troops is a negligible difference in maneuverability? Not when the base number is 35-40, it isn't. That's a pretty big block.
And as I said in my edit, it's a squared exponential. That means while you do have diminishing returns at higher penalties, you get increasing returns until you hit the top of the bell curve (of course, in the case of Ld tests, it does flatline at the end because of Insane Courage). Either way, you ideally want to have people testing in the Ld 4-5 range, where the chances for success are very slim.
Last but not least, I'm preparing for an opponent to counter this strategy by bringing several characters with magic weapons in different units, in which case it's very likely that one Wraith will die and one of your Core troopers will be forced to step up and be exposed to attacks from enemy rank-and-file. This is exactly when the champion comes in, which helps cap enemy CR from wounds at +3, and also makes that difference in CR generated by your attacks that much more valuable, because it could be the difference.
And as I said in my edit, it's a squared exponential. That means while you do have diminishing returns at higher penalties, you get increasing returns until you hit the top of the bell curve (of course, in the case of Ld tests, it does flatline at the end because of Insane Courage). Either way, you ideally want to have people testing in the Ld 4-5 range, where the chances for success are very slim.
Last but not least, I'm preparing for an opponent to counter this strategy by bringing several characters with magic weapons in different units, in which case it's very likely that one Wraith will die and one of your Core troopers will be forced to step up and be exposed to attacks from enemy rank-and-file. This is exactly when the champion comes in, which helps cap enemy CR from wounds at +3, and also makes that difference in CR generated by your attacks that much more valuable, because it could be the difference.