[RC-Monster Mike]

enlarging the bore on a hardened pinion is no trivial task. A lathe is the best tool for the job, along with specialized tooling(a normal drill bit will not make it through). Even if you were successful in opening the bore, you now have .5mm less meat at the set screw area, so it is much more likely to back out. Reducing the shaft is a much better idea and isn't terribly hard with a dremel and grinding wheel.