In device properties you have device block size setting. What text says is not feature but just note that default 128KB was increased to 256KB - by default people used to set this in past manually to 256KB themselves for performance reasons.
You can check what you use by checking what block size is set in each device property. If it says handler default, then it is default internal setting for that particular media (in past you could dump those with NSR_DEV_BLOCK_SIZE_MSG=YES variable). You can also see impact of various block sizes when using tape_perf_test command.