Set the Arg6 value to '-m256' (do not include the 'M' on the end).
At startup the default value of '48' generates a message like this in the backupserver errorlog:
"Dec 07 08:49:41 2013: Backup Server: 1.70.1.1: Configuring the maximum shared memory usage to 48 MB."
If the Arg6='-m256' is successful then you should see the following in your backupserver errorlog at startup:
"Dec 07 08:59:03 2013: Backup Server: 1.70.1.1: Configuring the maximum shared memory usage to 256 MB."
If you still have problems setting the shared memory parameter then a) export the backupserver's registry entries to a file and b) post said file contents to this discussion thread.