I do think the level 3 feature should expend Inspiration rather than just key to its size. But making a basic attack routine rely on that expenditure might be too pricey (especially when Valor could get the same weapon die size for free with a Shortsword/Rapier/Longsword2h/Greataxe respectively).
Personally I think I'm going to do an Oath of the Sacred Fist. I need to find where I put those 3.5 materials to see if I want to borrow any of the Sacred Fist PrC.