supposedly 7mm adjustment total - so depends on how the holes were drilled I suppose - If I understand correctly a 315 could go down to a 311 or up to a 319 or thereabouts - but I could be totally worng - not a shop rat.
In general, I have always heard dropping a shell size = remount - most likely good within a certain mondopoint (with exceptions, of course).
The post directly above mine was far more elegant