Quite nifty XP-trigger you created there, Arnie!
Kondi: yep, I also think that totally gimping a player's RP isn't really fun anymore if you are forced to just play with a horde of infantry units and at-guns.
I've been playing lately a custom campaign (Desert Rats) with a lot of AI units. As long as the AI has still RP available it's quite tricky to hold the line in defense. The AI can handle moving damaged units to the rear to make room for fresh troops at the front line. Give the AI enough resources and it will sooner or later run the player dry. That is when the challenge begins.
It all depends how many units the AI has of course and how it is scripted, otherwise the player can typically outnumber/flank the AI in every situation. Enough artillery for the AI can also make it quite difficult for players who are in the offensive. AI-arty is often lacking in scenarios.
Screenshot below: here is a triggered way to spawn additional units for specific difficulties. This way, you can keep it at original level on low-mid level and vastely increase it rather only on the higher levels. Besides unit spawning, you could also add in the same trigger more RP for the AI.
The advantage of this is that every difficulty tuning is tied to a single scenario-start trigger that you can disable anytime in the editor if you don't want to play with it. Original settings won't be touched by this.