I'm suddenly rewriting everything I had on psychic combat in DARK SUN as I had a revelation of sorts. It dawned upon me that the Weapon vs. AC Chart is the model I want to follow. Attack Mode corresponds to Weapon and Defense Mode to Armour Class. This retains the flavour of the old system, with strategic choices as to which Modes to employ with some better and some worse, while making the system mirror the existing Spellcraft & Swordplay physical combat system.
Hopefully ready to post tomorrow.