RMAN priorities

Whilst playing around with "_high_priority_processes” I ran across another hidden parameter called _rman_io_priority. Ostensibly this looks like another magic switch like _serial_direct_read to improve the speed of RMAN backups and restores.
But alas this parameter cannot be modified from its default value of 3

SQL> alter system set "_rman_io_priority"=6 scope=spfile;
alter system set "_rman_io_priority"=6 scope=spfile
*
ERROR at line 1:
ORA-00068: invalid value 6 for parameter _rman_io_priority, must be between 6
and 18446744073709551615

SQL> alter system set "_rman_io_priority"=7 ;
alter system set "_rman_io_priority"=7
*
ERROR at line 1:
ORA-00068: invalid value 7 for parameter _rman_io_priority, must be between 6 and 18446744073709551615

SQL> alter system set "_rman_io_priority"=18446744073709551614;
alter system set "_rman_io_priority"=18446744073709551614
*
ERROR at line 1:
ORA-02017: integer value required

The maximum value of an integer is 2147483647 or (2e31 -1)


SQL> alter system set "_rman_io_priority"=2147483647;
alter system set "_rman_io_priority"=2147483647
*
ERROR at line 1:
ORA-00068: invalid value 2147483647 for parameter _rman_io_priority, must be between 6 and 18446744073709551615

As this is an undocumented parameter I cannot raise an SR on this issue. My understanding is that this has been fixed in 11g although it still remains an undocumented parameter.