Hyperthetical question:
If you knew that Castle would replace your 6mm rotors regardless ( their error/ omission on the original listing ), would more of you be inclinded to have a go at grinding the shafts down from 6mm > 5mm ( all thats required is a vice/lathe & a dremel/file afterall, and it has been done by a few guys already with great success )?
Given the choice/dilemma I'd grind those shafts baby- 4 months is a long time to sit around whining about something you could fix is less than an hour with basic hand tools & little chance of messing it up ( little by little, check with calipers and/ or the pinion until it fits snuggly ).