thee, the drive side seat stay is not flattened (to the inside) where it meets the drop out? It's not simply chain rub if it is not flattened: the chain will not go into smallest cog at all.
And your frame was not used prior? Just asking because seems unlikely this issue would come up now on a 6 year old frame.
Also, 0.5mm would not have solved the unflattened aspect of the seat stay problem that I experienced.